β-Cyclodextrin coated lithium vanadium phosphate as novel cathode material for lithium ion batteries

Abstract

As a new carbon source, β-cyclodextrin was used to synthesize a Li₃V₂(PO₄)₃/C cathode material for lithium ion batteries (LIBs) via a rheological phase method. X-ray diffraction (XRD) patterns demonstrated that the sample had a pure monoclinic structure and sharp diffraction peaks, indicating good crystallinity. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the sample had a uniform and optimal particle size, which is beneficial to the electrochemical performance of LIBs. In the voltage range of 3.0–4.3 V, the initial discharge capacity of the sample was 111 mA h g⁻¹ and remained 109.3 mA h g⁻¹ after 50 cycles at 0.1C rate. In the high voltage range of 3.0–4.8 V, the initial discharge capacity was 151.4 mA h g⁻¹ and remained 131.5 mA h g⁻¹ after 25 cycles at 0.1C rate, indicating that β-cyclodextrin is a promising carbon source for LIB materials.

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1. Introduction

Lithium-ion batteries (LIBs) are being developed as low-cost and environmentally friendly systems to power an increasingly diverse range of applications.¹ The cathode material of LIB plays a critical role in determining its electrochemical performance,² and are usually based on layered structure LiMO₂ (M = Co,³ Ni,⁴ Mn⁵), spinel structure LiMn₂O₄ (ref. 6) and olivine structure LiFePO₄.⁷ In recent years, polyanion-type cathode materials such as LiMPO₄ (M = Co,⁸ Ni,⁹ Mn¹⁰) and Li₃M₂(PO₄)₃ (M = Fe,¹¹ V^12–17) have exhibited great potential for use in high power LIBs owing to their high power density, long cycle life, and good safety property.^18,19 Among them, monoclinic Li₃V₂(PO₄)₃ (LVP) is considered to be a promising cathode material besides LiFePO₄.²⁰

However, LVP has poor Li⁺ conductivity like other phosphate cathode materials,²¹ which limits its application in power-demanding environments such as electric vehicles (EVs).²² To solve this problem, a great deal of effort has been made to modify LVP through approaches such as carbon coating, doping, and the preparation of special morphologies.^23–26 Carbon coating can improve the electronic conductivity of LVP and reduce its particle size.¹⁰ Reducing particle size of LIB materials can decrease the lithium ion diffusion and electron transport distances, which is beneficial to rate capability.²⁷ Furthermore, carbon materials can be easily synthesized and derived from a wealth of sources,²⁸ including citric acid,^29–31 maleic acid,³² ascorbic acid,^33,34 humic acid,³⁵ stearic acid,³⁶ glycine,³⁷ sucrose,^38,39 glucose,⁴⁰ maltose,⁴¹ crystal sugar,^42,43 chitosan,⁴⁴ PVA (polyvinyl alcohol),^45,46 PEG (polyethylene glycol),¹ pitch,²² and EDTA (ethylene diamine tetraacetic acid), which we have previously reported on ref. 47.

Cyclodextrins (including β-cyclodextrins and γ-cyclodextrins) are products of the enzymatic hydrolysis of starch.⁴⁸ They are 7-membered sugar macrocycles composed of glucose monomeric units joined by α-1,4-linkages.⁴⁹ They also have a slightly cone-shaped hollow cylinder structure and can form molecular complexes with a variety of inorganic and organic compounds. In fact, they have been widely used in lithium battery research as a surfactant to effectively disperse solid substances in liquid and as an agent to promote complexation reactions, which is beneficial to material dispersion and molding. They can also act as binders because of their low cost, low resistance, strong bonding strength, and high stability in electrolyte. Cyclodextrins used as a carbon source for the synthesis of lithium ion battery materials are expected to convert to porous carbon during the pyrolysis process and inhibit LVP particle growth.⁵⁰ The reduced particle size will not only be helpful to shorten the lithium ion diffusion path of the LVP, but also improve the reaction efficiency of the LVP by increasing its specific surface area. Additionally, cyclodextrin-derived pyrolytic carbon has a better degree of graphitization, which would effectively improve the electrical conductivity and lithium ion diffusion rate of LVP, further improving its electrochemical performance.

At present, research on LVP is mainly concentrated in material synthesis and modification. Among the traditional and new preparation methods used to synthesize LVP, the rheological phase method⁵¹ has drawn particular attention as a new soft synthetic method. In rheological phase reaction systems, solid particles are uniformly distributed in a liquid substance as two coexisting phases according to their stoichiometry. The close contact between the two phases can effectively utilize their interfaces.⁵² Thus, the material and energy exchange between them can be carried out quickly and easily.⁵³ The reacted materials are then dried, pretreated, and annealed under high temperature to obtain the corresponding products. Rheological phase methods only require a low synthesis temperature and short annealing time to distribute the material uniformly and obtain enhanced electrochemical properties.

Herein, LVP/C cathode materials for lithium ion batteries were successfully synthesized via a rheological phase method using β-cyclodextrin (β-CD) as a new carbon source, and their structure and electrochemical behavior were characterized.

2. Experimental section

All reagents used in this work were of analytical grade and used without further purification. Li₂CO₃ (AR, 97%) was purchased from Beijing Chemical Reagents (Beijing, China). V₂O₅ (AR, 99%) was obtained from Shanghai Chemical Reagents (Shanghai, China). NH₄H₂PO₄ (AR, 99%) and β-cyclodextrin (AR, 99%) were purchased from Beijing Chemical Works (Beijing, China).

2.1 Materials synthesis

As shown in Scheme 1, LVP composites were prepared via a rheological phase method using Li₂CO₃, V₂O₅, NH₄H₂PO₄, and β-cyclodextrin as reagents. The proportion of β-cyclodextrin added to the reaction system was 0, 10, 15, or 20 wt%. The materials were weighed according to molar ratios and ground in a mortar for about 0.5 h to obtain a uniform mixture. Deionized water was then added dropwise to form a rheological state. The rheological mixture was heated in a sealed container at 80 °C for 12 h, then a green porous precursor was obtained. The precursor was ground and further dried at 60 °C for 4 h before annealing. In the annealing process, the precursor was initially annealed at 350 °C for 3 h in a flowing argon atmosphere to obtain a black material. The black material was then pressed in a bead machine for 10 min under the pressure of 8 MPa and further annealed at 750 °C for 6 h to obtain the final LVP samples.⁵⁴

Scheme 1 Schematic diagram of the Li₃V₂(PO₄)₃/C synthesis process.

2.2 Characterization

Thermo-gravimetric analysis (TGA; TG/DTA6200, EXSTAR, Japan) was used to optimize the pyrolysis temperature of the precursor powders under nitrogen flow, and the sample was heated from 20 to 900 °C at the rate of 10 °C min⁻¹. X-ray diffraction (XRD) measurement was used to detect the crystal purity and structure of the samples on a Bruker D8 Advance X-ray diffractometer (2θ = 10–60°) with a Cu Kα radiation source (λ = 1.5406 Å). JADE software was used to determine the phase composition with reference to corresponding JCPDS cards. Scanning electron microscope (SEM; FEI QUANTA6000) and high-resolution transmission electron microscopy (HRTEM; JEM-2010(HR), JEOL, Japan) was used to investigate the particle morphology of the samples. Brunauer–Emmett–Teller (BET; ASAP 2020) method was used to test the specific surface area, pore volume and pore size distribution of the uncoated and coated samples using nitrogen adsorption and desorption isotherms.

2.3 Electrochemical measurements

The electrochemical performance of the prepared samples was investigated in CR2025 coin type cells assembled with a lithium metal anode, a microporous polypropylene separator (Celgard 2400) and a cathode. The cathode was comprised of the prepared sample, polyvinylidene fluoride (PVDF), conductive carbon black (weight ratio of 8 : 1 : 1), and a proper amount of N-methyl-2-pyrrolidone (NMP) solvent. The cathode was rolled on an aluminum foil to form a thin sheet with uniform thickness and further dried at 80 °C for 12 h. The electrolyte was 1 M LiPF₆ in EC (ethylene carbonate) : DMC (dimethyl carbonate) (1 : 1, vol%). The cells were assembled in a glove box under a high-purity argon atmosphere. The galvanostatically charged/discharged curves were tested on a Land CT2001A electrochemical analysis instrument. Cyclic voltammograms (CVs) were conducted on a CHI660A electrochemical workstation. Electrochemical impedance spectroscopy (EIS) experiments were employed over a frequency range of 0.01 Hz to 100 kHz.

3. Results and discussion

3.1 TG analysis of the precursor powder

To optimize the pyrolysis temperature, thermo-gravimetric analysis (TGA) was carried out on the precursor powders with different β-CD coating amount. As shown in Fig. 1, three distinct weight loss regions were found, at 20–250, 250–350 and 350–900 °C. Below 250 °C, the samples exhibited a slight weight loss because of the release of adsorbed water. As the temperature was increased from 250 to 350 °C, the weight loss of the samples increased obviously, mainly owing to the pyrolysis and carbonization of the coating layer. Therefore, the pre-sintering temperature was set at 350 °C. This temperature not only satisfies the requirements of carbonization, but is also optimal for crystal growth. The weight loss of the samples from 350 to 900 °C increased gradually, mainly because of the occurrence of oxidation reduction reactions and the formation of the LVP crystals. The weight loss from 700 to 900 °C was particularly small, which indicates the formation of the crystals.

Fig. 1 Thermo-gravimetric analysis (TGA) curves of the precursor powders recorded from 20 to 900 °C at 10 °C min⁻¹ in N₂ atmosphere.

It also can be seen from Fig. 1 that the LVP/C samples showed an apparent weight loss in comparison with the uncoated sample. This indicates that the β-CD not only formed a carbon layer on the LVP, but also took part in the reaction of the raw materials. The pyrolytic carbon derived from the β-CD was also helpful in crystallizing the products. Additionally, the higher the β-CD content, the more the weight loss, because of the higher amounts of escaped CO₂ and H₂O.

3.2 Structural characterization

Fig. 2 shows the XRD patterns of the coated and uncoated samples obtained at the same final calcination temperature of 750 °C. The samples were both identified as LVP with a monoclinic structure based on their diffraction peaks at 20.62°, 24.30°, and 29.32°, which agree with the previous reports.^29,44,47 No diffraction peaks of carbon were observed owing to either its amorphous structure or low content.²² The pattern of the coated sample had obviously sharper diffraction peaks than those of the uncoated one, indicating that LVP/C had a better crystallinity.

Fig. 2 XRD patterns of the coated and uncoated materials prepared at the same calcination temperature of 750 °C (A). The structure of Li₃V₂(PO₄)₃ (B).

The lattice parameters of the coated and uncoated LVP samples fitted by JADE software are shown in Table 1. The coated sample had larger lattice parameters, which is beneficial for increasing the surface area of the material. An increased surface area would help the LVP/C particles to maintain better contact with the electrolyte and reduce the loss of their active materials. Therefore, the coated sample had advantageous structure, which was expected to improve its electrochemical properties. The increased surface area, pore volume and pore size of the coated sample can be verified using BET method (Table 2).

Table 1 Lattice parameters of the uncoated and coated LVP samples prepared at a calcination temperature of 750 °C

Samples	a/Å	b/Å	c/Å	V/Å^3
Uncoated	8.5861	12.0815	8.5764	889.65
Coated	8.5934	12.0833	8.5791	890.82

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Table 2 Pore properties of the uncoated and coated LVP samples

Samples	Surface area/m^2 g^−1	Pore volume/cm^3 g^−1	Pore size/nm
Uncoated	2.4282	0.0058	4.3878
Coated	7.1039	0.0187	10.5323

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3.3 Morphological characterization

Fig. 3(A) and (B) are SEM images depicting the morphology of the LVP/C and pristine LVP samples. The morphology of the LVP/C sample clearly differed from that of the pristine LVP sample. The primary particles of the uncoated LVP sample had a diameter of about 2 μm, but had a nonhomogeneous size distribution and irregular shape. In contrast, the LVP/C particles were spheroidal with a clear outline and had a homogenous distribution with smaller size (about 300 nm). This indicates that the β-CD inhibited the growth of the material and reduced its particle size. As seen from Fig. 3(B), the agglomeration of the uncoated LVP sample was serious and the secondary particles had a diameter of 12 μm. Fig. 3(A) shows that the introduction of β-CD resulted in relatively minimal agglomeration and a more uniform size distribution with much smaller secondary particle size and some small clusters (300 nm),⁴⁷ which is smaller compared with “several hundred nanometers in size” or “between 0.5 and 1 μm” reported by other authors.^34,40

Fig. 3 SEM pictures of the two samples prepared at the same calcination temperature of 750 °C: (A) LVP/C coated with 15% β-cyclodextrin, (B) pristine LVP.

Further surveys of the LVP/C particles were carried out by TEM. Fig. 4(A) shows that the LVP/C particles were spherical and uniformly coated with integral carbon layers. These carbon layers may have limited the growth of the particles. The magnified image in Fig. 4(B) shows that the carbon layers (about 50 nm in thickness) were amorphous.

Fig. 4 TEM images of LVP/C sample coated with carbon derived from β-cyclodextrin (A) and the magnified image (B).

Accordingly, β-CD coating made the LVP particles smaller, more uniform, and more homogeneous. These changes were expected to improve the electrochemical performance of the LVP/C composites by providing shorter diffusion paths and faster diffusion along grain boundaries.

As indicated by the above results, β-CD played an important role in the synthesis process. On one hand, β-CD acted as complexing agent and surface active agent, forming a complex with the metal ions and speeding up the reaction process during the formation of the rheological phase. Thus, the solid materials dispersed uniformly in the liquid to form the rheological body. On the other hand, β-CD acted as a reducing agent, deoxidizing V⁵⁺ after the pyrolysis and carbonization to form the LVP crystals. It is more meaningful that the β-CD existed as a network on the surface of the LVP samples, which was expected to be beneficial to their conductivity.

3.4 Electrochemical properties

As presented in Fig. 5(A), the initial charge and discharge curves of the LVP/C sample at 0.1C exhibited three charge flat plateaus (3.60, 3.67, and 4.09 V) and three corresponding discharge plateaus (4.06, 3.65, and 3.57 V), reflecting the phase transition processes among Li_xV₂(PO₄)_3(x=3–2.5), Li₂V₂(PO₄)₃, and LiV₂(PO₄)₃.⁵⁵ This also indicated that the two Li ions extraction/insertion behavior was reversible during the charge/discharge electrochemical reaction. In comparison, the flat plateaus of the pristine LVP sample were slightly lower than those of the coated one. From 3.0 to 4.3 V, the LVP/C sample showed a good performance with an initial capacity of 111 mA h g⁻¹ during discharge and 121 mA h g⁻¹ during charge, and its columbic efficiency was 91.7%. In contrast, the initial capacity of the pristine LVP sample was only 99.7 mA h g⁻¹ during discharge and 105.8 mA h g⁻¹ during charge. Therefore, the LVP/C sample had higher capacity and better reversibility. Fig. 5(B) showed the initial charge curves of the samples at 0.1C at the potential range from 3.0 to 4.8 V. Another inclined plateau appeared, which reveals the difficulty of the extraction of the third Li⁺. However, the LVP/C sample still exhibited excellent properties, with an initial capacity of 151.4 mA h g⁻¹ during discharge and 185.1 mA h g⁻¹ during charge, close to the theoretical capacity (197 mA h g⁻¹).⁵⁶ Furthermore, its columbic efficiency reached 81.79%. The initial capacity of the pristine LVP sample was only 117.1 mA h g⁻¹ during discharge and 151.4 mA h g⁻¹ during charge with a columbic efficiency of 78%. This demonstrated that the capacity of the LVP/C was effectively improved by β-CD coating.

Fig. 5 Initial charge–discharge profiles of the coated and uncoated samples at 0.1C in cut-off potential ranges of 3.0–4.3 V (A) and 3.0–4.8 V (B); cycling behaviors of the coated and uncoated samples at 0.1C at the range of 3.0–4.3 V (C) and 3.0–4.8 V (D).

Fig. 5(C) and (D) show the cycling performances of samples at 0.1C. It is clear that coating with β-CD significantly improved the cycle performance of the samples. In the potential range of 3.0–4.3 V, the LVP/C sample had an initial discharge capacity of 111 mA h g⁻¹ and could maintained at 109.3 mA h g⁻¹ after 50 cycles with a capacity retention of 98.46%. The pristine LVP sample could only maintained at 87.5 mA h g⁻¹ after 50 cycles with a capacity retention of 87.76%. When the potential range was set from 3.0 to 4.8 V, the initial discharge capacity of the LVP/C sample was 151.4 mA h g⁻¹ and decreased to 109.3 mA h g⁻¹ after 25 cycles. In contrast, the pristine LVP sample started with a lower discharge capacity of 117 mA h g⁻¹ and quickly decreased to 94 mA h g⁻¹ after 25 cycles. Therefore, β-CD greatly improved the electrochemical performance of LVP in both the low and high cut-off potential ranges, which was comparable with other reports.^{30,34,36,40–42,45}

The rate behavior of the coated and uncoated LVP materials was investigated in cut-off voltage range of 3.0–4.3 V under different discharge rates (0.2C, 0.5C, 1C, 2C) to observe the performance of the samples at high discharge rate, and to determine whether the samples have structure changes after returning to low current.⁵⁴ As can be seen in Fig. 6, the β-CD coated sample showed superior rate performance. Its initial discharge capacity was 113.7 mA h g⁻¹ at 0.2C and hardly decreased after 5 cycles. When the rate was increased to 0.5C, it showed a similar capacity of 111.6 mA h g⁻¹ and a good cyclic stability. Then the rate was increased to 1C and 2C after every 10 cycles and both of the coated and uncoated LVP samples were observed to have a decrease in capacity, especially the uncoated one. As the discharge rate returned to 0.2C after 35 cycles, the capacity of the β-CD coated sample maintained at 110.8 mA h g⁻¹ with a capacity retention of 97.44%, while that of the pristine sample was only 93.8 mA h g⁻¹ with a capacity retention of 93%. These results further prove that the excellent cycle performance and rate properties of the LVP/C material were related to the use of β-CD as a carbon source.

Fig. 6 Rate performance of the coated and uncoated samples at a range of 3.0–4.3 V.

The CV curves were presented in Fig. 7. Obviously, both samples exhibited three couples of charge/discharge plateaus and an additional charge plateau, corresponding to three two-phase transition behaviors and a solid solution behavior as the y in Li_yV₂(PO₄)₃ changed (y = 3.0, 2.5, 2.0, 1.0, 0). The involved reactions are given in eqn (1)–(4) below.^54,57,58

Li₃V₂(PO₄)₃ ↔ Li_2.5V₂(PO₄)₃ + 0.5Li⁺ + 0.5e⁻ (1)

Li_2.5V₂(PO₄)₃ ↔ Li₂V₂(PO₄)₃ + 0.5Li⁺ + 0.5e⁻ (2)

Li₂V₂(PO₄)₃ ↔ LiV₂(PO₄)₃ + Li⁺ + e⁻ (3)

LiV₂(PO₄)₃ ↔ V₂(PO₄)₃ + Li⁺ + e⁻ (4)

Fig. 7 Cyclic voltammograms of the coated and uncoated samples at 0.1 mV s⁻¹.

The oxidation peaks located around 3.629 V worked with step (1) and 3.698 V worked with step (2). Both of the peaks were correlated to the removal of first Li⁺. The second Li⁺ was extracted in a single step around 4.117 V (3). Simultaneously, eqn (1)–(3) also associated with the transformation of V from V³⁺ to V⁴⁺. Eqn (4) was corresponded to the extraction of the third Li⁺ at around 4.589 V, associated with the transformation of V from V⁴⁺ to V⁵⁺.^22,29 The three reduction peaks were associated with the re-insertion of Li⁺ ions. Additionally, the extraction of the third Li⁺ was a solid solution behavior that is hard to reverse. Therefore, there was no reduction peak corresponding to the oxidation peak at 4.589 V. Furthermore, the curve for the LVP/C sample exhibited sharper peaks and smaller ΔV values than that of the pristine sample, which reveals the improvement in the reversibility after β-CD coating.

EIS measurements were carried out and are presented in Fig. 8. The straight lines in low frequency were due to the diffusion behavior of Li⁺ within the electrode particles, i.e., the Warburg impedance.⁴⁰ The depressed semicircles in high frequency were attributed to the charge transfer resistance at the electrode/electrolyte interface and the corresponding capacitances. Both the resistances were decreased after the addition of β-CD, indicating that carbon coating with β-CD was beneficial to the extraction/insertion of the Li⁺, and highlighting again the improvement in the diffusion coefficient and conductivity after carbon coating.

Fig. 8 Electrochemical impedance spectra of the coated and uncoated samples.

4. Conclusion

As a new carbon source, β-cyclodextrin was used to synthesize a Li₃V₂(PO₄)₃/C cathode material for lithium ion batteries via a rheological phase method. XRD patterns demonstrated that the sample had a pure monoclinic structure and good crystallinity. SEM and TEM images showed that the thickness of the carbon layer was about 50 nm and that the particles had a uniform and optimized size (about 300 nm), which greatly improved the electrochemical performance of the battery. When the voltage range was set from 3.0 to 4.3 V, the initial discharge capacity of the sample was 111 mA h g⁻¹ and remained 109.3 mA h g⁻¹ after 50 cycles at 0.1C. In the high voltage range of 3.0–4.8 V, the initial discharge capacity of the sample was 151.4 mA h g⁻¹ and remained 131.5 mA h g⁻¹ after 25 cycles at 0.1C. The LVP/C sample also showed good rate performance, good cycle stability, and low resistance, demonstrating that β-cyclodextrin is a promising carbon source for LIB materials.

Acknowledgements

This work was supported by the National Key Research and Development Program of China “New Energy Project for Electric Vehicle” (2016YFB0100204), the National Natural Science Foundation of China (21373028), the Joint Funds of the National Natural Science Foundation of China (U1564206), Major achievements Transformation Project for Central University in Beijing, Beijing Science and Technology Project (D151100003015001) and Construction project of Beijing University Engineering Research Center (2016093902).

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