Synthesis of biocarbon coated Li₃V₂(PO₄)₃/C cathode material for lithium ion batteries using recycled tea

Abstract

A biocarbon coated Li₃V₂(PO₄)₃/C (LVP-C) cathode material was synthesized by a facile sol–gel method using recycled tea as both the structural template and biocarbon source. X-ray diffraction (XRD) patterns show that LVP has a monoclinic structure with space group P2₁/n. High-resolution transmission electron microscopy (HRTEM) images show that the LVP nanoparticles are surrounded by amorphous biocarbon, and the thickness of the biocarbon shell is about 10–20 nm. Electrochemical measurements demonstrate that the LVP-C nanocomposite shows a significantly better rate capability and cycling performance than pure LVP. In the potential range of 3.0–4.3 V, the LVP-C nanocomposite delivers a high initial discharge capacity of 132 mA h g⁻¹ at 0.5 C, and maintains an initial discharge capacity of 110 mA h g⁻¹ at 10 C. After 80 cycles at 10 C, it still retains a discharge capacity of 110 mA h g⁻¹. Electrochemical impedance spectroscopy (EIS) measurements have disclosed that the LVP-C sample exhibits enhanced electrode reaction kinetics and improved electrochemical performance. The good electrochemical performance of the LVP-C nanocomposite is mainly related to the presence of the conductive biocarbon, thus leading to an improvement in the electron and lithium ion diffusivity. These results indicate that the biocarbon coated LVP-C material is a promising candidate for large capacity and high power cathode materials in next generation lithium-ion batteries for electric vehicles.

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Introduction

With rapid social development, vehicles are becoming more and more important in our daily life. However, in the past few years, with the depletion of non-renewable energy resources such as oil and natural gas, looking for new sources of energy for vehicles has become one of the most important questions that the humanity has to face. Electricity, as a non-polluting clean energy, has become the first energy choice for vehicles. Electricity can be converted from three major renewable sources: wind, solar and hydroelectric, and they are all clean energies. It is meaningful to apply electric energy to vehicles in order to restore the increasingly damaged environment . At the same time, it is quite necessary to select a suitable electric energy storage device. At present, rechargeable lithium-ion batteries (LIBs) are considered as one of the most important energy storage systems for the ever growing demand for portable products and electric vehicles, owing to their lightweight nature, high energy density, and durable cycling life.^1,2

A LIB mainly consists of a negative electrode (anode), a positive electrode (cathode), a separator and an electrolyte (shown in Fig. 1). Nowadays, most LIBs use LiCoO₂ as the main cathode material due to its simple synthesis, low irreversible capacity loss, and good cycling performance.^3,4 However, its high cost, safety issues and toxicity limit its further use in large-scale applications, such as electric vehicles and hybrid electric vehicles. Therefore, it is imperative to develop novel cathode materials for LIBs.

Fig. 1 Schematic illustration of a lithium ion battery employing graphite as anode and monoclinic Li₃V₂(PO₄)₃ as cathode.

Recently, lithiated transition-metal phosphates, such as LiMPO₄ (M = Mn, Fe, Co, Ni) and Li₃M₂(PO₄)₃ (M = Ti, V, Fe), have attracted great interest as potential cathode materials for LIBs, because they have good electrochemical and thermal stability, competitive energy density, and high operating potentials.^5–8 Among these phosphates, monoclinic Li₃V₂(PO₄)₃ (LVP) is one of the most promising cathode materials due to its high theoretical capacity (197 mA h g⁻¹), high operating voltage (up to 4.0 V), and safety.^9,10 In the potential range between 3.0 and 4.8 V, the reversible cycling of all three lithiums from LVP would correspond to a theoretical capacity of 197 mA h g⁻¹, which is the highest of all phosphates reported for cathode materials.^11,12 In the potential range between 3.0 and 4.3 V, a reversible capacity of 131 mA h g⁻¹ can still be obtained.¹³ Furthermore, monoclinic LVP with slightly distorted VO₆ octahedra and PO₄ tetrahedra in the NASICON structure provides more efficient three-dimensional pathways for Li⁺ extraction and re-insertion than the one-dimensional pathways in LiFePO₄.¹⁴ Luckily, LVP is a fairly well studied material for lithium battery cathodes. Recently, Rui et al.¹⁵ presented a review specifically on the recent development of monoclinic LVP cathode materials for LIB applications. Host structure, mechanism of lithium insertion/extraction, transport properties, synthetic methods and electrochemical properties in terms of rate capability and cyclic stability were summarized and analyzed. An insight into the future research and development of LVP compound was also discussed.

However, similar to other lithium transition metal phosphates, the poor intrinsic electronic transport of LVP limits its applications.^9,13 In order to improve the electronic transport of LVP, many conductive materials have been employed to build a conductive layer for the phosphates.^16–18 Among these conductive materials, carbon is characterized by a fast electron transport and regarded as an excellent coating material.^19–22 Carbon coatings are usually realized by introducing an organic precursor in the starting materials. The organic precursor can be converted into electronically conductive carbon through a pyrolysis process at high temperature under inert atmosphere. The carbon coats the particles to form a conducting LVP-C composite electrode material.²³ Additionally, carbon can act as a reducing agent and thus simplify the atmospheric conditions in the synthesis.²⁴ Carbon-coated LVP can promote the contact between particles, decrease the polarization of the electrode materials and eventually improve the performance of LIBs. Carbon sources are very diverse, Rui et al.¹⁵ summarized the effect of different types of carbon coatings on the electrochemical performance of LVP cathodes. LVP using 3.98 wt% citric acid as carbon source could deliver a discharge capacity of 110.8 and 97.9 mA h g⁻¹ at 5 C and 10 C in the potential region of 3.0–4.3 V.²⁵ LVP with 16.4 wt% baker’s yeast cells as the carbon source could deliver a discharge capacity of 100.5 mA h g⁻¹ at 5 C in the potential region of 3.0–4.3 V.²⁶

The carbon coating plays a key role in improving the rate capability and cycling retention of electrode materials. A variety of biocarbon sources have been used for the development of high performance lithium-ion batteries (LIBs) such as baker’s yeast cells, tobacco mosaic virus, adenosine triphosphate, bacteria, lotus pollen grains, viruses, microalgae, rice husk and crab shells.²⁷ Tea is one of the most popular drinks in the world, especially in China. However, some tea is very expensive, such as Longjing tea. Throwing it away after drinking it once or twice is really wasteful. It will be a meaningful thing if it could be recycled. As far as we know, researchers usually choose organic or polymer matter as carbon sources, here we have choosen a new carbon source (used tea), which has never been used as carbon source before. Different from ordinary carbon sources, tea is a biocarbon source coming from the nature. There are different metal cations in tea that can improve the electronic conductivity of LVP to some extent through cation doping.

Compared to solid-state reactions, in solution methods the starting materials can be mixed at a molecular level, which is conducive to an homogeneous reaction.²⁸ In this work, we employed a facile sol–gel method to prepare a biocarbon coated LVP-C cathode material using recycled tea as both the structural template and biocarbon source, introducing the biocarbon to the surface of LVP particles by the thermal decomposition of used tea under nitrogen atmosphere. Herein, the biocarbon is not only providing a network to restrict the growth and agglomeration of the LVP crystallites, but it is also supplying a high electronically conductive layer which is expected to enhance the electrochemical performance of LVP.²⁹

Experimental

Sample preparation

The reagents used in this work were NH₄VO₃ (99%, Tianjin Bodi Chemical Co. Ltd.), C₂H₂O₄·2H₂O (99.5%, Tianjin Bodi Chemical Co. Ltd.), NH₄H₂PO₄ (99%, Tianjin Bodi Chemical Co. Ltd.), Li₂CO₃ (97%, Tianjin Guangfu Fine Chemical Research Institute), and used tea leaves (Jasmine tea, Fujian, China). Distilled water was used during the synthesis of LVP-C. The LVP-C samples were prepared using the biotemplate (recycled tea) with a sol–gel method. The typical synthetic process is shown in Fig. 2. The recycled tea was placed in an air oven at 100 °C for 10 h, then it was ground into powder in a mortar. The tea powder used for the experiments was isolated by centrifugation, washed with distilled water and dried. Oxalic acid (3.80 g) and NH₄VO₃ (2.34 g) in stoichiometric ratios were dissolved in 100 mL deionized water with magnetic stirring at 70 °C. Oxalic acid was used here not only as a chelating reagent but also as a reducing agent. After a clear blue solution of VOC₂O₄ was formed (eqn (1)), tea powder (2.00 g) was added to the blue solution with magnetic stirring at 70 °C. After stirring vigorously for 2 h, a mixture of stoichiometric NH₄H₂PO₄ (3.45 g) and Li₂CO₃ (1.18 g) was added to the solution while stirring at 70 °C until the formation of sol, and then a gel formed in an air oven at 100 °C. The obtained dark green product was sintered at 700 °C for 8 h in flowing nitrogen (eqn (2)), and the final product was a black powder. LVP samples without tea powder were prepared by the same process for comparison. The two samples were named LVP (without tea powder) and LVP-C (2.00 g tea powder), respectively.

Fig. 2 Schematic illustration of the fabrication process of LVP-C.

Sample characterization

X-ray diffraction (XRD) patterns of the samples were measured using a PANalytical X’Pert PRO X-ray diffractometer (Netherlands) with Cu Kα radiation in order to identify the phase composition. The diffraction patterns were collected over a diffraction angle 2θ range of 10°–60°, with an acquisition time of 12.0 s at 0.02° step size. Fourier transform infrared spectroscopy (FT-IR) was performed using a Nicolet Nexus spectrometer (Nicolet, NEXUS 470, USA) by using the KBr wafer technique in order to study the composition of the recycled tea. Infrared spectra were recorded in the region 4000–450 cm⁻¹, with a resolution of 4.00 cm⁻¹. Thermogravimetric analysis (TGA) of the samples was conducted in air at a heating rate of 10 °C min⁻¹ from 45 °C to 850 °C using a thermal analyzer (TGA1 STAR System) in order to study the carbon content. High-resolution transmission electron microscopy (HRTEM) was carried out on a Philips FEI TF20 microscope working at 200 kV. The procedure for the preparation of the HRTEM samples is as follows: firstly, the cathode material (0.03 g) was ground using ethyl alcohol for 5 h in a mortar, then the cathode material was placed in a beaker (50 mL) containing 30 mL ethyl alcohol, and the supernatant was used for the HRTEM test after 12 hours.

Preparation of electrodes and electrochemical testing

The charge–discharge performance of the samples was evaluated using LIR2032 coin cells. The cathode materials were prepared by mixing the sample with acetylene black and polyvinylidene fluoride (PVDF) in a weight ratio of 8 : 1 : 1 in N-methyl pyrrolidone (NMP) to ensure homogeneity. Here, acetylene black is the conductive agent, PVDF is the binder, and NMP is the solvent. After NMP was evaporated, the mixture was coated on aluminum foil, then the aluminum foil was dried under air atmosphere at 80 °C for 5 h and under vacuum atmosphere at 120 °C for 10 h and then cut into circular strips of 15 mm in diameter. The mass calculation of the active materials was carried out based on the 80% of difference between the mass of active LVP-C material coated Al foil and the mass of the pristine Al foil. The coin cells were assembled in a glove box filled with high-purity argon, where lithium metal was used as the anode, a polypropylene film as the separator, and 1 M LiPF₆ in an electrolyte consisting of ethylene carbonate/dimethyl carbonate/ethylene methyl carbonate in a volume ratio of 1 : 1 : 1. The charge–discharge performance of the synthesized samples was tested using a Channels battery analyzer (CT3008W) at different current densities between 3.0 and 4.3 V cut-off voltages using the coin cells. The electrochemical impedance (EIS) and cyclic voltammetry (CV) measurements were performed using a PARSTAT 2263 electrochemical workstation. EIS was recorded with a frequency ranging from 100 kHz to 10 mHz and a AC signal of 5 mV in amplitude as the perturbation. The voltage range of the CV measurements was 3.0–4.3 V and the scanning rate was 0.1 mV s⁻¹. All the tests were performed at room temperature.

Results and discussion

Structural analysis and morphology characterization

LVP crystallizes in two different forms: a rhombohedral structure and a thermodynamically more stable monoclinic structure.³⁰ Fig. 3 shows the XRD patterns of pure LVP and the LVP-C composite.

Fig. 3 XRD patterns of (a) LVP-C composite and (b) pure LVP.

It can be seen that both patterns can be indexed to a monoclinic structure with space group P2₁/n, consistent with those previously published.^13,30 Furthermore, no diffraction peaks of carbon are observed, which may be attributed to the amorphous state of the biocarbon coating. The sharp diffraction peaks of both samples indicate good crystallinity and the intensity of the diffraction peaks is obviously strengthened by the biocarbon coating. The lattice parameters of pure LVP and the LVP-C composite were calculated by Jade5, and the results are listed in Table 1. It can be seen from Table 1 that the lattice parameters of LVP do not suffer large changes as a consequence of the biocarbon coating, and this suggests that the presence of biocarbon does not affect the structural properties of LVP. Moreover, there is no notable peak shifting after the introduction of biocarbon.

Table 1 Lattice parameters of pure LVP and the LVP-C sample

Samples	a (Å)	b (Å)	c (Å)	β (°)	V (Å^3)
LVP	8.51	12.06	8.58	89.42	881.57
LVP-C	8.53	12.04	8.59	89.90	884.21

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The FTIR spectrum of the recycled tea is shown in Fig. 4. The broad band at 3500 cm⁻¹ is ascribed to the O–H stretching vibration of water. The dominant bands near 1652 and 1534 cm⁻¹ are assigned to the amide I and amide II groups of the proteins in the recycled tea. The band at 2920 cm⁻¹ is derived from the CH₂ asymmetry stretching vibration of proteins and carbohydrates in the tea. The band at 1047 cm⁻¹ is ascribed to the C–O stretching vibration of the carbohydrates found in the RNA, DNA, cell membrane and cell wall of tea cells.³¹

Fig. 4 FTIR spectrum of the recycled tea.

The carbon content of the samples was determined by thermogravimetric analysis as shown in Fig. 5. According to Rui et al.,²⁴ the composites initially undergo a weight loss in the temperature range of 300–500 °C, which is attributed to the removal of carbon by oxidation into gaseous products (CO or CO₂). Above 500 °C, LVP gains weight due to the oxidation of V(III) to higher valence states in air. According to the weight loss step on the TGA curves, the carbon content of pure LVP is very small and can be neglected, while the carbon content of LVP-C is about 5.0%.

Fig. 5 TGA curves of pure LVP and the LVP-C composite.

To further investigate the microstructure of the samples and the biocarbon coating on the LVP particles, HRTEM images of pure LVP and LVP-C powder particles were obtained. Fig. 6c, taken from a section of Fig. 6a, shows that there is hardly any amorphous carbon around pure LVP particles. This result is consistent with the carbon content analysis (Fig. 5). The small amount of amorphous carbon may come from the pyrolysis of oxalic acid at high temperatures under inert atmosphere. Fig. 6d, taken from a section of Fig. 6b, shows that LVP particles are surrounded by amorphous biocarbon, and that the highly crystalline particle is coated with an amorphous biocarbon shell of about 10–20 nm. It is necessary to stress that the thickness of the biocarbon shell can be controlled by the quantity of tea powder. The presence of the biocarbon layer can impede the grain growth of LVP particles and improve the electronic conductivity of the LVP cathode material. Fig. 6c and d both display clear lattice fringes with a d-spacing of 0.365 nm corresponding to the (121) plane of monoclinic LVP, which is consistent with the XRD analysis.

Fig. 6 HRTEM images of the samples. (a and b) Low magnification images, showing structures of pure LVP (a) and LVP-C (b), respectively. (c) An enlarged image of a partial area in (a). (d) An enlarged image of a partial area in (b).

Formation mechanism of LVP-C

Fig. 7a shows the structure of tea leaves.³² The hydrophilic anion groups in the biomacromolecules of tea leaf particles (Fig. 7b) can improve the electrostatic interaction between the tea powder surface and cations and regulate the particle nucleation and precipitation.³³ When VOC₂O₄, Li₂CO₃, and NH₄H₂PO₄ are added into the tea particle solution, the Li⁺, VO²⁺ and V³⁺ cations combine with the negatively charged OH⁻ groups in the structure of tea leaf, and self-assemble on the tea leaf surface by electrostatic interactions (Fig. 7c). Then, the biocarbon coated LVP-C cathode material is synthesized by heat treatment (700 °C) in a reducing atmosphere (N₂) (Fig. 7d). Fig. 7d represents how LVP particles are surrounded by an active biocarbon network after heat treatment of the precursor, which is consistent with the HRTEM image in Fig. 6d. The biocarbon not only enhances the conducting properties but also restrains the particle growth during the sintering process, and hence significantly improves the electrochemical performance of LVP.^13,34 This structure can help to explain why the LVP-C sample can be used at very high rates in lithium ion batteries.

Fig. 7 (a) Structure of tea leaf, (b–d) formation mechanism of biocarbon coated LVP-C.

Electrochemical properties

Fig. 8a and b show the initial charge–discharge curves of pure LVP and LVP-C in the potential range of 3.0–4.3 V at different rates, respectively. It is necessary to point out that the cell was firstly charged at a constant current density to a potential of 4.3 V, then charged at a constant voltage (4.3 V) to a minimum current value (0.001 mA), and then discharged at a constant current density. It can be seen that all the charge–discharge curves show a similar shape presenting three charge–discharge plateaus that correspond to the three types of reversible phase transformation between Li_3−xV₂(PO₄)₃ (x = 0, 0.5, 1.0, and 2.0). Fig. 8a shows that the initial discharge capacity of pure LVP is about 115, 92, 71, and 52 mA h g⁻¹ at 0.1, 0.5, 1, and 2 C, respectively. Fig. 8b shows that the initial discharge capacity of LVP-C is about 124, 132, 131, 128, 121, and 110 mA h g⁻¹ at 0.1, 0.5, 1, 2, 5, and 10 C, respectively. Compared to the above mentioned LVP cathodes reported by Rui et al.,¹⁵ it is easy to conclude that the LVP-C sample synthesized with the tea powder biotemplate has a larger range of capacity and smaller potential differences of the plateaus than pure LVP, indicating that the LVP-C sample has a lower electrochemical polarization which leads to a better electrochemical performance in charge–discharge processes. Fig. 8c shows the rate capability and cycling performance of pure LVP and LVP-C.

Fig. 8 (a) The initial charge–discharge curves of pure LVP at 0.1, 0.5, 1, and 2 C, respectively. (b) The initial charge–discharge curves of LVP-C at 0.1, 0.5, 1, 2, 5, and 10 C, respectively. (c) The rate capability and cycling performance of pure LVP and LVP-C. (d) The cycling performance of LVP-C at 10 C.

Compared to pure LVP, LVP-C shows a stable capacity at each state (0.1, 0.5, 1, 2, and 5 C), while the capacity of pure LVP decays very quickly, especially at high rates. The capacity of LVP-C at 0.1 C is much lower than that of 0.5, 1 and 2 C and is comparable to 5 C. The increase in the capacity at 0.5, 1 and 2 C can be attributed to the activation of the LVP-C cathode material after several cycles, a fuller contact between LVP-C particles and the electrolyte, and the formation of more lithium ion transport channels. Fig. 8d shows the cycling performance of LVP-C at 10 C. The discharge capacity of LVP-C is still maintained at 110 mA h g⁻¹ after 80 cycles. Just as expected, the LVP-C sample exhibits a better rate capability and cycling performance than pure LVP, mainly attributed to the existence of the biocarbon framework, which can improve the electronic conductivity of the composite and accelerate the electron transport.

CV measurements

For LVP, two lithium ions can be easily extracted/inserted reversibly between 3.0 and 4.3 V based on the V³⁺/V⁴⁺ redox couple.³⁵ To verify this electrochemical behavior, Fig. 9 shows the CV curves of pure LVP and LVP-C at a scan rate of 0.1 mV s⁻¹ in the voltage range of 3.0–4.3 V. It can be seen from Fig. 9 that the CV curves of pure LVP and LVP-C are very similar. They both have three oxidation peaks (around 3.68, 3.77, 4.21 V) and three corresponding reduction peaks (around 3.88, 3.51, 3.43 V) between 3.0–4.3 V, which are consistent with lithium ion extraction/insertion during the phase transition processes: Li₃V₂(PO₄)₃ ↔ Li_2.5V₂(PO₄)₃ ↔ Li₂V₂(PO₄)₃ ↔ LiV₂(PO₄)₃, and are also in good agreement with the charge–discharge curves in Fig. 8. However, there are some differences between the two samples. The curve of LVP-C has better-defined peaks than that of pure LVP, which demonstrates that LVP-C displays outstanding reversibility of the lithium extraction/insertion reactions.³⁶

Fig. 9 The CV curves of pure LVP and LVP-C at a scan rate of 0.1 mV s⁻¹ in the voltage range of 3.0–4.3 V.

EIS measurements

To better understand the electrochemical kinetic properties of the samples, EIS was carried out after 100 cycles at a charge of 4.3 V. Fig. 10a shows the Nyquist plots of the materials. It can be seen that both samples have similar EIS curves. The small intercept is related to the solution resistance (R_s), the depressed semicircle at medium frequencies represents the charge-transfer resistance (R_ct) and the double-layer capacitance (C_dl), and the sloping line at low frequencies is attributed to the Warburg impedance associated with the diffusion of lithium ions in the electrode (Z_w).³⁷ An equivalent circuit model is shown in the inset of Fig. 10a for the analysis of the impedance spectra. From Fig. 10a, the R_ct value of the LVP-C sample is much smaller (74.8 Ω) than that of pure LVP (199.5 Ω), demonstrating that the electrons can be transferred more easily between LVP particles and the electrolyte for the LVP-C sample as a result of its biocarbon network.³⁸ Fig. 10b shows the linear fitting of Z_real vs. ω^−1/2 in the Warburg region.

Fig. 10 (a) Nyquist plots of pure LVP and LVP-C. (b) Relationship plot of impedance as a function of the inverse square root of the angular frequency in the Warburg region.

In comparison with pure LVP, the LVP-C sample has a lower slope, indicating the higher lithium ion diffusion coefficient of the sample.³⁹ Thus, the LVP-C sample prepared using recycled tea as both the structural template and biocarbon source exhibits enhanced electrode reaction kinetics and improved electrochemical performance.

Conclusions

In summary, LVP-C nanoparticles covered with amorphous biocarbon have been successfully synthesized by a facile sol–gel method using recycled tea as both the structural template and biocarbon source. Compared to pure LVP, the LVP-C nanocomposite exhibits much improved electrochemical performance, particularly at high current rates. The results can be attributed to the conductive biocarbon framework of the LVP-C nanocomposite. The experimental results demonstrate that biocarbon coating is an efficient route to prepare LVP-C with good electrochemical performance. Furthermore, the proposed approach also reminds us that the combination of biological and chemical aspects has excellent prospects in the field of electrochemistry.

Acknowledgements

The authors thank the Natural Science Foundation of China (Grant no. 51272144, 51472127 and 51172132) for financial support. They also thank Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass and Functional Ceramics for technological support.

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