3D-porous β-LiVOPO₄/C microspheres as a cathode material with enhanced performance for Li-ion batteries

Abstract

3D porous β-LiVOPO₄/C microspheres were synthesized through a solvothermal method followed by a post-heat treatment. TG-DSC and FTIR results illustrate crystal structure transformation from α→β-LiVOPO₄. XRD results reveal pristine and synthesized powders that were crystallized in the triclinic α-LiVOPO₄ and orthorhombic β-LiVOPO₄ phase, respectively. Scanning electron microscopy (SEM) and pore distribution results reveal that β-LiVOPO₄/C spheres were built from small nanoplates and pores with a wide diameter distribution. HRTEM results indicate encapsulation of β-LiVOPO₄/C particles with amorphous carbon shells. A porous β-LiVOPO₄/C cathode delivered 134 mA h g⁻¹ and 74 mA h g⁻¹ initial discharge capacities at 0.1 C and 1 C, respectively. The cell presented superior capacity retention attributed to the contributions of surface coating, high specific surface area, and porous architecture that serve as facile electrical conduits for ion/electron transport.

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1. Introduction

Phosphate polyanion compounds are of considerable interest because of their ability to intercalate/deintercalate lithium. Among these compounds, vanadium phosphates are deemed as interesting Li-intercalation hosts because of the feasibilities of vanadium existing with three oxidation states (III, IV and V).^1–4 The vanadium phosphate cathodes, namely, LiVOPO₄,^5,6 VOPO₄,^7,8 LiVPO₄F,^9–12 and Li₃V₂(PO₄)₃,^13–16 display stable frameworks for the next-generation high-power lithium-ion batteries. The relatively high voltage LiVOPO₄ has been regarded as one of the most suitable candidate cathode because of its high energy density, thermal stability, environmental friendliness, and low raw material cost.¹⁷

Two kinds of LiVOPO₄ exists, α-LiVOPO₄ (triclinic, space group P1̄) and β-LiVOPO₄ (orthorhombic, space group Pnma). The orthorhombic-phase has been extensively investigated because of its excellent ion-intercalation properties. β-LiVOPO₄ crystals possess a three-dimensional network consisting of interconnected VO₆ octahedral and PO₄ tetrahedral units that consequently create channels for Li⁺ ion movement.¹⁸ Nevertheless, the Li-ion movement is limited by poor electronic conductivities and sluggish lithium diffusion.^19,20 Thus, improvements in Li intercalation and de-intercalation performances are strongly required. Recently, enhanced Li-storage capacities in this cathode (LiVOPO₄) have been demonstrated by the strategies of nano-sizing and mixing electrical additives.^21,22 Alternatively, morphology tailoring, carbon nano-coating, and porous structures have been also shown to be capable of improving electrochemical properties in Li-ion battery electrode.^23,24 Porous structures and efficient wetting of the active materials by electrolytes are believed to facilitate Li-ion transport for the electrochemical reactions.²⁵

As a result, product morphology appears to be one of the important factors that improve LiVOPO₄ kinetic properties. Hollow spheres of electroactive α-LiVOPO₄ were synthesized by K. Saravanan et al. via solvothermal method shown superior electrochemical properties.¹⁷ Furthermore, enhanced storage capacities α-LiVOPO₄/C with porous structure was successful prepared at 500 °C in air by B. J. Paul et al. Nevertheless, porous sphere of β-LiVOPO₄/C has not been studied or reported yet in the previous studies. Based on these findings, we demonstrate a simple solvothermal method that can synthesize a 3D porous β-LiVOPO₄/C sphere cathode material by a post-heat treatment, and investigate the properties of this material.

2. Experimental

2.1 Preparation of sample

The procedure for formulating the porous sphere β-LiVOPO₄/C composite is schematically illustrated in Fig. 1. First, α-LiVOPO₄ composite was hydrothermally synthesized using stoichiometric raw materials such as NH₄VO₃, LiNO₃, and (NH₄)₂HPO₄. Solution I was formulated by completely dissolving NH₄VO₃ and (NH₄)₂HPO₄ through stirring at 80 °C. Solution II was prepared by dissolving the surfactant poly-vinylpyrrolidone (PVP), the composite template, and LiNO₃ in distilled water. Subsequently, oxalic acid and ascorbic acid were added as reducer, respectively. The pH of solution II was adjusted to 4.0–4.5 using ascorbic acid. Solutions I and II were mixed and stirred for 1 h. The concentration was kept at 0.1 mol L⁻¹. Finally, the prepared solution was transferred in Teflon-lined stainless steel reaction vessel and the vessel was sealed tightly. The mixture was autoclaved at 300 °C for 20 h in an oven and was cooled to ambient temperature. The black precipitate α-LiVOPO₄ composite was obtained and dried under vacuum. The pristine α-LiVOPO₄ was calcined at 500 °C for 4 h in air to allow volatilization of CO and CO₂ from the degradation of organic compounds during the calcination process. The synthesized β-LiVOPO₄/C was obtained through the transformation of crystal structure from α→β-LiVOPO₄. The 3D porous β-LiVOPO₄/C sample was likewise obtained.

Fig. 1 Schematic illustration of synthesis process for 3D porous β-LiVOPO₄/C sphere composite.

2.2 Sample characterization

Thermal analysis was performed by a thermo-gravimetric differential scanning calorimetry (TG-DSC) apparatus (STA-449C, Netzsch, Germany) at a heating rate of 5 °C min⁻¹ under ambient atmosphere. Structural and crystalline phase analyses of the products were performed for the powder via Cu Kα radiation X-ray diffraction (XRD, Rint-2000, Rigaku). The Fourier transform infrared (FTIR) spectrum was obtained using Nicolet 460 FTIR spectrophotometer. Elemental carbon analysis of sample was performed by C–S analysis equipment (Eltar, Germany). The samples were observed by scanning electron microscopy (SEM) (JEOL, JSM-5600LV) and Tecnai G12 transmission electron microscope (TEM). Sample pore distribution was measured by nitrogen adsorption–desorption measurements (QuadraSorb Station).

2.3 Electrochemical measurements

The electrochemical characterizations were performed using CR2025 coin-type cell. The synthesized composite was mixed with acetylene black and poly-vinylidenedifluoride (PVDF) according to 80 : 10 : 10 weight ratio in NMP. The cathode was prepared by spreading the above mentioned mixture on aluminum foil. Charge–discharge tests of the samples were performed in coin cells with cathodes and lithium anodes. After solvent evaporation, the electrodes were punched in the form of 14 mm diameter disks and dried under vacuum at 120 °C for 4 h. The test cell consisted of the positive electrode and the lithium foil negative electrode separated by a porous polypropylene film, and 1 mol L⁻¹ LiPF₆ in 1 : 1 : 1 EC : EMC : DMC as the electrolyte. The coin cells were assembled in a glove box filled with dry Ar. The cells were charged in CC-CV mode and discharged in CC mode over a voltage range of 3.0–4.5 V against Li/Li⁺ electrode at room temperature. Cycling and charge–discharge performances of the testing cells were carried out using an automatic galvanostatic charge–discharge unit and a LAND battery cycler. Cyclic voltammograms (CV) were recorded with differential scanning rate between 3.0–4.5 V at room temperature with a CHI660D electrochemical analyzer.

3. Results and discussion

The X-ray diffraction (XRD) patterns for the pristine and synthesized powder are shown in Fig. 2. The pattern of the pristine (Fig. 2a) is monoclinic LiVOPO₄ structure, which is consistent with the JCPDS data (#72-2253) and literature reports.^5,6 Nevertheless, the peaks of the α-LiVOPO₄ indicate the presence of a spot with amorphous structure. Some peaks that can be attributed to VO₂ impurity are observed in the α-LiVOPO₄ material. Fig. 2b shows the XRD pattern of the LiVOPO₄ powder formed through a post-treatment of calcination at 500 °C for 4 h in air. All of the peaks perfectly match the standard orthorhombic LiVOPO₄ (JCPDS #85-2438), indicating the high purity and high crystallinity of the obtained sample. The amount of carbon in the composite is approximately 2.1 wt% as determined by CS analysis method. Remaining carbon in the composite is not detected, which indicates that the residual carbon was amorphous.

Fig. 2 The X-ray diffraction (XRD) patterns for the pristine (a) and synthesized sample (b).

Thermo-gravimetry and differential scanning calorimetry (TG-DSC) spectra of the pristine powder are shown in Fig. 3. Weight loss took place in three main steps. The first step (I) occurred between room temperature and 360 °C. Weight loss was approximately 5.20%, which can be mainly attributed to the removal of adsorbed water, and the decomposition of oxalic acid and ascorbic acid compounds. In the second step (II), the weight loss of 5.56% between 360–500 °C can be attributed to the decomposition of residual citric compounds and PVP, corresponding to the exothermic peak at approximately 390 °C as shown in DSC curve. Two endothermic peaks are found in the vicinity of 490 °C and 580 °C. The first and second peaks correspond to α→β-LiVOPO₄ and β→α-LiVOPO₄ phase transformation, respectively. This trend of phase transformation with temperature has been reported in previous literature as well.¹⁸ Furthermore, no obvious weight loss between 500–580 °C can be found in the TG curve during the third step (III), which implies the synthesis of well-crystallized β-LiVOPO₄. A tiny peak of derivative weight loss after 580 °C in TG curve is observed, which may be attributed to the decomposition of some lithium compounds. The suitable temperature to synthesize porous β-LiVOPO₄/C material is 500 °C.

Fig. 3 Thermogravimetry and differential scanning calorimetry (TG-DSC) spectra of the pristine powder.

To investigate the differences between the pristine and post-treated sample, the produced FTIR spectrum shown in Fig. 4 was used. The peak at 1400 cm⁻¹ reveals the symmetric stretching vibration of carboxylate derived from oxalic acid compounds, and the peak at 1644 cm⁻¹ shows C=C stretching vibration of ascorbic acid compounds.²⁷ The peaks close to 2364 cm⁻¹ and 3265 cm⁻¹ can be attributed to C–O–C asymmetrical stretching vibration of CO₂ and O–H stretching vibration of H₂O, respectively. The CO₂ and H₂O were absorbed by PVP because of its high sensitivity in air.²⁸ After the post-heat treatment, the peak derived from oxalic acid, ascorbic acid, and PVP compounds disappeared except for the weak peak at 1640 cm⁻¹, which is a possible result of the symmetric stretching of phosphate. The first overtone bands of the phosphate stretching vibration reveal that all organic compounds have been consumed during the post-treatment.²⁹ Furthermore, the FTIR spectrum of β-LiVOPO₄ matches well with the published spectra.^30–32 Peaks at 1170, 1020, and 990 cm⁻¹ correspond to the asymmetric stretching vibration (ν₃) of the PO₄ tetrahedra, while the peak at 951 cm⁻¹ can be assigned to the symmetric stretching vibration (ν₁) of the PO₄ tetrahedra. In addition, the peaks at 570 cm⁻¹ (ν₄) and 418 cm⁻¹ (ν₂) reflect the bending vibration of the PO₄ tetrahedra. Furthermore, the peaks close to 902, 638, and 500 cm⁻¹ can be attributed to the V–O stretching and VO₆ octahedra bending vibrations.^33–35

Fig. 4 The FTIR spectroscopy of pristine (a) and synthesized sample (b).

The SEM images of pristine and synthesized powder and the TEM and HRTEM images of the latter are shown in Fig. 5. The SEM images present the sphere-like structure of β-LiVOPO₄/C the diameters of which range from 10 to 15 μm (Fig. 5b and d). Compared with the pristine powder (Fig. 5a), the surface of the β-LiVOPO₄/C particles can demonstrate the formation of a porous architecture (Fig. 5b) better because of the exhaust of gases generated during the oxalic acid anion, ascorbic acid anion, and PVP degradation and expulsion from the LiVOPO₄ structure.

Fig. 5 SEM image of α-LiVOPO₄ (a) and β-LiVOPO₄/C (b–d), and TEM (e) and HRTEM images of β-LiVOPO₄/C (f).

The magnified view shows the presence of abundant small nanometer to micrometer sized sphere structures (Fig. 5c–e), indicating the porous architecture that includes a wide diameter distribution and an interlaced pore system network.

Furthermore, the SEM images with higher magnification reveal that the β-LiVOPO₄/C spheres are built from small nanoplates with thicknesses ranging from 80–200 nm (Fig. 5b and c). The nanoplates were stacked and attached interpenetratively on the spherical surface, with different sizes of pores spread all over the sphere. By decreasing the lithium-ion diffusion and the electron transportation distance, the nanoplates can facilitate faster transport and better intercalation kinetics of lithium ions.^36,37 The α-LiVOPO₄ sphere was prepared successful in the presence of PVP during the hydrothermal treatment process. PVP was speculated to behave as a template. The PVP vinyl groups are hydrophobic while carbonyl groups are hydrophyllic, which results in the formation of micelles.^38,39 As a surfactant, PVP with amphiphilic characteristics was expected to adsorb on the surface of LiVOPO₄ nanoplates, and would not only prevent the agglomerations of nanoplates, but also be transformed into conductive amorphous carbon layer coated on LiVOPO₄ surfaces during the post-treatment step.⁴⁰ Under high pressure and temperature, the α-LiVOPO₄ composite sphere was formed by the nanoplates self-assembly.

The β-LiVOPO₄/C sample HRTEM image is shown in Fig. 5f. The β-LiVOPO₄/C particle is encapsulated with an amorphous carbon shell. The thickness of the carbon shell is approximately 2.5 nm. The lattice fringe of β-LiVOPO₄ has an interplanar spacing of 0.331 nm, which corresponds to the (2 0 1) lattice planes.

To derive more information of the porous characteristics, the N₂ adsorption–desorption isotherm was obtained using Barrett–Joyner–Halenda (BJH) method as shown in Fig. 6. The plot represents a type-IV curve that indicates the porous behavior of the β-LiVOPO₄/C sample.⁴¹ The pore distribution curve (Fig. 6 inset) suggests a wide pore diameter distribution varying from mesopore (2–100 nm) to macropore (>100 nm), as shown in the TEM images of Fig. 5e and c, respectively. The porous architecture is an advantage because it contributes to electrochemical reactions.⁴² The pores can be flooded with electrolyte, especially the ones inside, thereby ensuring that a high specific surface area is in contact with the electrolyte; consequently, high flux of lithium across the interface is produced, which provides higher electrochemical reaction kinetics for electrochemical reactions.^25,43 The specific surface area of β-LiVOPO₄/C is 5.77 m² g⁻¹, which is higher compared to that of the α-LiVOPO₄/C porous structure (4.81 m² g⁻¹) derived by B. J. Paul et al.²⁶

Fig. 6 N₂ adsorption–desorption isotherms of β-LiVOPO₄/C (inset: BJH pore size distribution plot).

The first charge–discharge curves of β-LiVOPO₄/C cell at different rates are shown in Fig. 7a. The initial charge capacities of β-LiVOPO₄/C at 0.1, 0.2, 0.5, 1, and 2 C (here 1 C = 166 mA g⁻¹) are approximately 156, 137, 118, 82, and 53 mA h g⁻¹, respectively, and the discharge capacities are 134, 128, 108, 74, and 46 mA h g⁻¹, respectively. The resulting Coulomb efficiency values of the initial cycle are 85.90%, 93.43%, 91.27%, 90.49%, and 88.11%. The reason for the higher initial charge capacity compared with the discharge capacity at 0.1 C is the cathode electrolyte interface.^44,45 The mean voltages of β-LiVOPO₄/C at 0.1, 0.2, 0.5, 1, and 2 C are 3.932, 3.927, 3.904, 3.811, and 3.734 V, respectively, indicating that the β-LiVOPO₄/C battery has high discharge plateau and low polarization at low rate. The electrochemical performance, as exemplified by the charge–discharge capacity at different rates, is as high as those of most related studies.^19,46,47 However, the discharge capacity decreases when the current rate increases mainly because of low intrinsic electronic conductivity and increased polarization upon cycling at high current density.^48,49 Based on these observations, our future work will involve improving the electrochemical performance at high current rates.

Fig. 7 Charge–discharge profiles at different rate (a) and cycling performance (b), and charge–discharge profiles in different cycle time at 0.1 C (c) of β-LiVOPO₄/C cathode.

To establish the lithium insertion reaction stability in the β-LiVOPO₄/C, the cycling performance of β-LiVOPO₄/C was investigated (Fig. 7b). The discharge capacity of the cell is 131 mA h g⁻¹ after 100 cycles, and the capacity retention is 97.8% of its initial discharge capacity. In addition, the cell retains 96.1%, 92.6%, 86.5%, and 84.8% of their initial discharge capacities at 0.2, 0.1, and 2 C, respectively, within 30 cycles. These results show slight improvement compared with those of earlier studies.^19,47,48 In particular, the discharge capacity of the cell has remained at 135 mA h g⁻¹, which resulted in 100.7% retention after another 30 cycles.

The charge–discharge curves of β-LiVOPO₄/C cell in the 1st, 50th, and 100th cycles at 0.1 C are shown in Fig. 7c. The specific discharge capacities are approximately 134, 132, and 131 mA h g⁻¹, respectively. The capacities among charge–discharge curves are similar and the voltage plateau is stable after the 50th and 100th cycles, indicating effective cycling performance.

Therefore, the porous β-LiVOPO₄/C sphere composite demonstrates a better lithium storage performance. The improved electrochemical performance and capacity retention may be attributed to the 3D porous architecture and the high specific surface area, as shown in Fig. 5 and 6.

Cyclic voltammetry (CV) curves of the prepared samples at scan rates of 0.1, 0.2, 0.5, 1.0 mV s⁻¹ between 3.0 V and 4.5 V are shown in Fig. 8a. With increased scan rate, the anodic peaks shift to higher potential and the corresponding cathodic peaks are lowered, resulting in the increase of polarization and irreversibility. The porous β-LiVOPO₄/C sphere composite shows good symmetrical shapes with increasing scan rate, indicating higher enhancement in electrode reaction reversibility and lower polarization of porous β-LiVOPO₄/C sphere composite compared with to those reported in related studies.^22,50

Fig. 8 Cyclic voltammetry curves for β-LiVOPO₄/C cathode at different scan rates (a) and CV in different cycle time at a scan rate of 0.1 mV s⁻¹ (b).

The CV curves in the 1st, 50th, and 100th cycle at 0.1 mV s⁻¹ scan rate are shown in Fig. 8b. Redox peaks for β-LiVOPO₄/C are observed, which is consistent with those of the charge–discharge tests at 0.1 C as shown in Fig. 7c. Fig. 8b shows the minimal change of redox peak positions after the 50th and 100th cycles, indicating slight polarization and good cycling performance.

4. Conclusions

In summary, 3D porous architecture orthorhombic β-LiVOPO₄/C microsphere composite was synthesized by calcining triclinic α-LiVOPO₄ as pristine material in the air atmosphere. The pore diameters varied from mesopore to macropore sizes. The initial discharge capacity of 3D porous β-LiVOPO₄ cathode was 134 mA h g⁻¹ at 0.1 C and the cell presented remarkable cycling stability. The enhanced performance can be attributed to the high surface area and the wide distribution range of pore structures.

Acknowledgements

This study was supported by National Natural Science Foundation of China (Grant no. 51302324 and 51272290) and the Fundamental Research Funds for the Central Universities of Central South University (2013zzts028).

Notes and references

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