Monoclinic Li₃V₂(PO₄)₃/C nanocrystals co-modified with graphene nanosheets and carbon nanotubes as a three-dimensional-network cathode material for rechargeable lithium-ion batteries

Abstract

A graphene nanosheet and carbon nanotube co-modified Li₃V₂(PO₄)₃/C composite has been first prepared via a hydrothermal-assisted sol–gel method. The crystal structure, morphology and electrochemical performance of the as-prepared sample are studied in detail. In this material, the Li₃V₂(PO₄)₃/C (100–150 nm) nanoparticles are strongly adhered to the surface of the graphene nanosheets while the carbon nanotubes are embedded into the Li₃V₂(PO₄)₃/C and graphene nanosheets, and both carbon additives have interlaced to form a crosslinked three-dimensional (3D) mixed conducting network. Thus, the composite exhibits a remarkably high rate capability and long cycle stability, delivering an initial discharge capacity of 147.5 mA h g⁻¹ at 20C within a voltage range of 3.0–4.8 V and the capacity retention is 82.7% after 2000 cycles. With a desirable cell performance, this approach offers a new idea in design and preparation of electrode materials for lithium-ion batteries.

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Introduction

Nowadays, lithium-ion batteries are gaining rapidly growing attention due to the fast development of sustainable energies, electric vehicles (EVs) and hybrid electric vehicles (HEVs).^1–4 It is known that the cost, safety, cycling time and energy density are the critical issues for practical applications of lithium-ion batteries. Lithium cobalt oxide (LiCoO₂)⁵ was the first compound to be utilized as a cathode material for commercial lithium-ion batteries. However, the high cost, toxicity and poor electrochemical performance at high current rate prohibit its use in large-scale applications. Recently, several materials such as lithium manganese oxide (LiMn₂O₄),^6,7 lithium nickel cobalt manganese oxide (LiNi_1/3Co_1/3Mn_1/3O₂)⁸ and lithium iron phosphate (LiFePO₄)^9–11 have been proposed as the next generation electrodes for lithium-ion batteries because of their good thermal stability. In 2002, Nazar's group reported the crystal structure and electrochemical characteristics of monoclinic (space group P2₁/n) lithium vanadium phosphate (Li₃V₂(PO₄)₃).¹² It has been investigated as one of the promising cathode materials because an average 4.0 V extraction/reinsertion voltage can be obtained between 3.0 and 4.8 V and the higher theoretical capacity of 197 mA h g⁻¹.^13–15 What's more, the Li₃V₂(PO₄)₃ phase consists of a three-dimensional (3D) framework of slightly distorted VO₆ octahedra and PO₄ tetrahedra sharing oxygen vertexes, which houses Li-ions in relatively large interstitial sites, leading to fast ionic transport.^16,17

Unfortunately, just like LiFePO₄,⁹ the pure Li₃V₂(PO₄)₃ electrode suffers from a poor electronic conductivity and a low ionic conductivity, which strongly limits its high rate performance. Yin et al.¹⁸ reported that the electronic conductivity of Li₃V₂(PO₄)₃ is about 10⁻⁸ S cm⁻¹, while the Li-ions diffusion coefficiency in Li₃V₂(PO₄)₃ ranged from 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹, as reported by Rui et al.¹⁹ In the past few years, various synthesis and processing approaches, such as reducing the particle size,^12,20,21 doping an appropriate amount of other alien ions^22–26 and coating with conductive carbon,^27–30 have been adopted to enhance the conductivity. Among the various methods, carbon coating is an economic and feasible technique that is widely used to improve the electronic conductivity. Nevertheless, the uniformity of carbon coating is crucial to achieve high electronic conductivity and Li-ion transfer because the thick carbon layer may block the Li-ions diffusion through the electrode/electrolyte interface. Thus, effective carbon coating with Li₃V₂(PO₄)₃ is an important factor for the high discharge capacity and good cycle stability, especially at high current rate.

Graphene nanosheet, as an outstanding representative of two-dimensional (2D) crystals, offers superior electrical conductivity and high surface area^31,32 and has been used to modify the electrode materials for lithium-ion batteries. Compared with the traditional carbon decomposed from glucose,³³ sucrose,³⁴ citric acid,³⁵ oxalic acid,³⁶ polyethylene glycol³⁷ and so on, graphene nanosheet is an effective additive for cathode materials. Some electrode materials, such as Li₃V₂(PO₄)₃,^38–41 has been incorporated with graphene to achieve greatly enhanced electrochemical performance. It is found that carbon coating is still necessary, as it not only acts as a conductive additive improving the electronic conductivity of Li₃V₂(PO₄)₃, but also forms a barrier to prevent the direct contact between the Li₃V₂(PO₄)₃ and electrolyte, hence minimizing the vanadium dissolution into the electrolyte.⁴² Apart from graphene nanosheet, one-dimensional (1D) carbon nanotube has also attracted a great deal of attention for energy storage applications owing to its good electrical properties, superior mechanical and thermal stability.^43,44 Recently, there are several reports about using the carbon nanotube as carbon source to improve the electrochemical performance of LiCoO₂, LiFePO₄ or Li₃V₂(PO₄)₃.^45–47 Zhang group⁴⁷ reported that the multi-walled carbon nanotubes-doped Li₃V₂(PO₄)₃ was successfully synthesized by a microwave assisted sol–gel method, and the obtained composite displays high rate capability, excellent reversibility and lower charge-transfer resistance. These reveal that one-dimensional (1D) carbon nanotube is an ideal conductive additive in the cathode systems, and modifying cathode materials with carbon nanotube might be promising to improve their overall electrochemical performances.

The combination of two-dimensional (2D) graphene and one-dimensional (1D) carbon nanotube into three-dimensional (3D) graphene/carbon-nanotube hybrids is considered as one of the most effective strategies to modify the electrode materials for lithium-ion batteries.^48–50 The introduction of one-dimensional (1D) carbon nanotube can inhibits the aggregations of three-dimensional (3D) graphene nanosheet⁵¹ and controls the intergraphene sheet distance.⁵² In 2013, Zhou's group⁵⁰ has synthesized the graphene and carbon nanotube modified LiFePO₄ nanocomposite via a facile single step polyol reducing method. In this composite, the mixed conducting network of graphene and carbon nanotube not only provides more paths but also accelerates the speed for both electron conduction and ion diffusion, which lead to the significant enhancement of specific capacity and cycling performance.⁵⁰ However, the effects of graphene nanosheet and carbon nanotube doping on the electrochemical performance of Li₃V₂(PO₄)₃/C have not been reported. Consequently, this paper is focused on the study of Li₃V₂(PO₄)₃/C doped with graphene and carbon nanotube as cathode material for lithium-ion batteries.

In this study, we attempt to design and prepare the carbon-coated Li₃V₂(PO₄)₃ nanocrystals further modified by graphene nanosheet and carbon nanotube through a hydrothermal-assisted sol–gel method. The residual carbon from the decomposition of oxalic acid was found to effectively bridge the graphene nanosheet and carbon nanotube for forming a conductive network. Due to the multiple conductive routes and network, both electrons and Li-ions diffusion are expected to be accelerated throughout the whole Li₃V₂(PO₄)₃ nanoparticles. For comparison purpose, the samples modified only with carbon or graphene nanosheet were also prepared and studied.

Experimental

Preparation of the samples

Nanosized Li₃V₂(PO₄)₃/C composite (abbreviated as C-LVP) was prepared through a hydrothermal-assisted sol–gel method²⁷ (shown in Fig. 1(a)). In a typical synthesis, stoichiometric of Li₂CO₃ (A.R.), NH₄VO₃ (A.R.) and NH₄H₂PO₄ (A.R.) were dissolved in deionized water with magnetic stirring at room temperature (25 °C). An appropriate amount of oxalic acid was then added to the above solution and magnetically stirred for 1 h. After a clear blue solution formed, the mixture was transferred into a 100 mL Teflon-lined autoclave and then kept at 180 °C for 48 h. Before sol–gel treatment, the hydrothermal procedure was introduced to form a partially carbonized layer, which could limit the increase of the particles size during the calcination process. The resulting brown mixture was heated gently with continuous stirring to evaporate the excess water at 80 °C until a gel was formed. Finally, the dried gel was preheated at 350 °C for 3 h and afterwards calcined at 750 °C for 6 h under a flow of N₂ to obtain the C-LVP. The carbon content in the C-LVP sample was maintained at 5 wt%.

Fig. 1 Schematic illustration of preparing C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) composites via a hydrothermal-assisted sol–gel route.

Graphene nanosheet (2 wt%) modified Li₃V₂(PO₄)₃/C composite (abbreviated as C-LVP/GNS) was prepared using the same route (see Fig. 1(b)). Moreover, the graphene nanosheet (1 wt%) and carbon nanotube (1 wt%) co-modified Li₃V₂(PO₄)₃/C (abbreviated as C-LVP/(GNS+CNT)) was also prepared by a hydrothermal-assisted sol–gel method (see Fig. 1(c)). It should be pointed out that the carbon contents in both two final samples were about 5 wt%.

Structure and morphology characterization

The crystal structure of the as-prepared samples was examined by X-ray diffraction (XRD) using Rigaku 2200 powder diffractometer with Cu-Kα radiation. The X-ray diffraction patterns were collected for 4 s at each 0.02° step width from 15 to 55°. The morphology was investigated by using a scanning electron microscope (SEM, Hitachi-X650 microscope) and transmission electron microscopy (TEM, JEOL-2000CX). The amount of carbon element in the samples was measured by elemental analyzer (Elementar VarioEL III, Germany). The Raman spectrum was measured by a Renishaw (Dongwoo DM500i, Japan) in ViaRaman microscope at room temperature (25 °C) with the 532 nm line of an Ar ion Laser as the excitation source.

Electrochemical characterization

The electrochemical performance of the as-prepared C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) as cathode materials was evaluated using a CR2025 coin-type cells. The working electrodes were fabricated by dispersing active materials, acetylene black and poly(vinylidene fluoride) (PVDF) in a weight ratio of 85 : 10 : 5 in N-methylpyrrolidone (NMP) solvent to form a homogeneous slurry, followed by plastering the slurry on an aluminum foil and then dried in vacuum oven at 100 °C for 12 h. All electrodes were cut into disks with a diameter of 14 mm, the average mass loading of which was about 2.5 mg cm⁻². For electrochemical measurements, half cells were assembled in an Ar-filled glove box using the above prepared disks as cathode, lithium metal as the negative electrode and a polypropylene film (Celgard 2500) as the separator and 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) with a 1 : 1 : 1 volume ratio as the electrolyte. The galvanostatic charge and discharge experiments were performed at room temperature (25 °C) by using a LAND-CT2001A battery system (Wuhan Jinnuo Electronics Co., Ltd) at different rates within the voltage range of 3.0–4.8 V. The measurement of electrochemical impedance spectroscopy (EIS) was conducted on a CHI660A electrochemical workstation (Chenhua, China) at room temperature (25 °C) with sinusoidal signal of 5 mV over the frequency range from 100 KHz to 10 mHz. Cyclic voltammetry (CV) was conducted on the above workstation at a scan rate of 0.1 mV s⁻¹ in the voltage range of 3.0–4.8 V.

Results and discussion

The XRD patterns of the as-prepared C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) samples together with that of standard LVP (JCPDS card no. 80-1515) are presented in Fig. 2. According to Fig. 2, all diffraction peaks seen from the three samples can be indexed to monoclinic LVP with a space group of P2₁/n, which indicate that the introduction of C, GNS and CNT have no effect on the structure of LVP. Moreover, no obvious peaks corresponding to C, GNS and CNT are observed in the XRD patterns owing to their low contents. The results are in good agreement with these previous reports.^{12,27,39,40,47} In all, the XRD results prove that the hydrothermal-assisted sol–gel route is a good method to prepare LVP-based electrode materials without any discernible impurities.

Fig. 2 XRD patterns of C-LVP, C-LVP/GNS, C-LVP/(GNS+CNT) and the standard LVP.

Fig. 3 shows the SEM images of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) samples, which present the morphological difference of particles. It can be clearly seen from Fig. 3(a and d) that the size of C-LVP particles are mostly distributed from 100 to 200 nm. The small particle is benefit for the electrochemical performance.⁵³ For the C-LVP/GNS sample (as shown in Fig. 3(b and e)), the particle size is smaller than that of C-LVP. GNS is responsible for the formation of nanosized morphology during the hydrothermal-assisted sol–gel process, owing to the GNS can prevent the growth and aggregation of C-LVP, and the result is good consistent with the previously reported papers.^38,53 According to Fig. 3(c and f), the C-LVP particles with an average diameter of about 120 nm are homogeneously grown on the GNS/CNT network, in which highly dispersed CNT are interconnected with the conductive GNS.

Fig. 3 SEM images of the as-prepared samples: (a and d) C-LVP, (b and e) C-LVP/GNS and (c and f) C-LVP/(GNS+CNT).

The morphologies of the as-prepared samples were further characterized by TEM shown in Fig. 4. It is noted that the particles of C-LVP show a wide size distribution ranging from 100 to 200 nm with some agglomeration (see Fig. 4(a)). Besides, a thin carbon layer (several nanometer) decomposed from oxalic acid is coated on the surface of LVP particles (Fig. 4(d)), which is classified to be effective in improving the electrochemical performance of the electrode. With a thicker carbon coating, the reversible capacity tends to decrease partly because of the increasingly hindered electrolyte transport.⁵⁴ After 2 wt% GNS added into the C-LVP, it can be found from Fig. 4(b and e) that the LVP nanocrystals coated with carbon are embedded in the GNS which is clearly visible. And this may facilitate the fast charge transport through the highly conductive GNS.⁵⁵ For C-LVP/(GNS+CNT) (1 wt% GNS, 1 wt% CNT), as shown in Fig. 4(c and f), it is obvious that CNT with a diameter of about 10–20 nm are well connected with GNS to form a mixed conductive network, which leads to electronic continuity between C-LVP particles. It should be mentioned that the residual carbon in C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) are all about 5 wt%, determined by element analysis. The detailed results are given in Table 1.

Fig. 4 TEM images of (a and d) C-LVP, (b and e) C-LVP/GNS and (c and f) C-LVP/(GNS+CNT), showing an overview of the micro–nano structure of the composites.

Table 1 Carbon contents in C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) samples tested by element analysis

Carbon content (wt%)	C-LVP	C-LVP/GNS	C-LVP/(GNS+CNT)
Total	5.03	4.97	5.01
GNS	0	2	1
CNT	0	0	1

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As illustrated in Fig. 2, there is no obvious peaks corresponding to carbon can be found in the XRD patterns. Nevertheless, the existing of carbon species in the C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) samples is demonstrated by the 532 nm Raman spectrum shown in Fig. 5. All the spectra of the three samples shows two characteristic bands of carbonaceous materials located at 1330 cm⁻¹ (D-band, disorder-induced phonon mode) and at 1600 cm⁻¹ (G-band, graphite band). The D band of carbonaceous materials is attributed to the A_1g mode that relates to the breakage of symmetry at the edges of graphite sheets, and the G band is due to the E_2g zone-center mode of crystalline graphite.⁵⁶ The symmetric PO₄ stretching vibration at 950 cm⁻¹ is not observed in the spectrum of the samples, which could be caused by the screening effect of carbon.⁵⁷ The decrease in the D/G intensity ratio of the C-LVP/(GNS+CNT) (from 0.97 for C-LVP to 0.94 for C-LVP/GNS, and then to 0.92) can be assigned to the decreasing edge defects. The above results indicate a relatively high degree of graphitization, which may lead to an improved electronic conductivity.^27,58

Fig. 5 Raman spectra of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) composites.

Fig. 6 presents the initial charge/discharge profiles and cycle performances of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) at a low rate of 0.1C between 3.0–4.8 V. For the three samples (see Fig. 6(a)), obviously, there are four plateaus in the charge process which corresponds to the extraction of three Li-ions from LVP over four two-phase transformation, while the corresponding discharge curves present three plateaus due to the solid state solution behavior of insertion of Li-ions into V₂(PO₄)₃.¹³ The detailed electrochemical reactions are summarised as follows: Li₃V₂(PO₄)₃ ↔ Li_2.5V₂(PO₄)₃, Li_2.5V₂(PO₄)₃ ↔ Li₂V₂(PO₄)₃, Li₂V₂(PO₄)₃ ↔ LiV₂(PO₄)₃ and LiV₂(PO₄)₃ ↔ V₂(PO₄)₃. The initial discharge capacity at 0.1C is 163.1, 182.7 and 190.3 mA h g⁻¹ for C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) samples, respectively. Obviously, the reversible capacity of C-LVP/(GNS+CNT) is higher than that of C-LVP/GNS, which is due to the three-dimensional (3D) conductive network for both electrons and Li-ions, promoting the fast charge/discharge process. Furthermore, as shown in Fig. 6(b), compared to C-LVP and C-LVP/GNS, the C-LVP/(GNS+CNT) also exhibits very stable capacity retention with a negligible capacity fade, for its discharge capacity dropped very little to 188.6 mA h g⁻¹ after 50 cycles.

Fig. 6 (a) Initial charge/discharge profiles and (b) cycle performances of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) at 0.1C in the voltage range of 3.0–4.8 V at room temperature (25 °C).

The rate capabilities of the as-prepared samples at different current rates between 3.0 and 4.8 V are shown in Fig. 7. For C-LVP (see Fig. 7(a)), the discharge capacities decrease rapidly and the polarization becomes bigger with the increasing current rates. It can be noted from Fig. 7(b) that the C-LVP/GNS sample exhibits better rate performance than C-LVP. This is because that GNS as a two-dimensional (2D) conductive layer can hinder the agglomeration of C-LVP during the calcination process, which could reduce the transport path lengths of electrons and Li-ions in C-LVP particles.⁵⁹ On the other hand, GNS offers a superior electrical conductivity which could enhance the electronic conductivity of C-LVP. As illustrated in Fig. 7(c), the C-LVP/(GNS+CNT) possesses better rate capacity and smaller polarization than C-LVP and C-LVP/GNS. In C-LVP/(GNS+CNT) sample, the one-dimensional (1D) CNT plays an important role in connecting the C-LVP and GNS, result in the formation of three-dimensional (3D) conductive network. Thus, even at high rate of 5 and 10C, it can still deliver discharge capacities of 166.2 and 154.6 mA h g⁻¹, respectively. The detailed discharge capacities of C-LVP/GNS and C-LVP/(GNS+CNT) in this paper and LVP/G based electrodes in other reports are summarised in Table 2.

Fig. 7 Electrochemical characteristics: (a–c) discharge profiles and (d) cycle performances of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) electrodes at various C-rates in 3.0–4.8 V at room temperature (25 °C); (e) the dependence of specific capacity on current density.

Table 2 Summary of the electrochemical performances of LVP/GNS based electrodes

Sample	Voltage (V)	Discharge capacity (mA h g^−1)						Reference
		0.2C	0.5C	1C	2C	5C	10C
C-LVP/GNS	3.0–4.8	178	171	160	146	130	115	This work
C-LVP/(GNS+CNT)	3.0–4.8	187	185	180	175	166	155	This work
C-LVP/G	3.0–4.8	—	—	169	165	161	146	39
LVP/G	3.0–4.8	—	177		137	117	—	41
C-LVP/G	3.0–4.8	174	166	157	138	116	—	42

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Fig. 7(d) gives the comparison of cycle performances for C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) samples at various current rates. The C-LVP shows an inferior cycle performance especially at high rates due to its low electronic conductivity as proved by the EIS results (see Fig. 9). After the addition of GNS, the specific capacity and capacity retention of the sample are remarkably improved. But for the C-LVP/(GNS+CNT) sample, it exhibits stable capacities at each state. It is worth noting that about 99.3% capacity is still remained at 10C after 10 cycles, much better than that of C-LVP/GNS (97%). This result is similar to that of CNT-modified GNS as high capacity anode material for rechargeable lithium-ion batteries reported by Honma's group.⁶⁰ Fig. 7(e) compares the rate performances of the samples at 0.2–10C. It can be found that the difference in capacity among C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) is small at low rates, but the gap increases with increasing the charge/discharge current density. This phenomenon reveals the advantage of using three-dimensional (3D) interpenetrating network structure to obtain an excellent electrochemical performance.

High rate performance of C-LVP/(GNS+CNT) electrode was further investigated. Fig. 8 shows the 1st and 2000th charge/discharge profiles and cycle performance of the electrode at 20C in 3.0–4.8 V. The C-LVP/(GNS+CNT) delivers a discharge capacity of 147.5 mA h g⁻¹ for the first cycle with a coulombic efficiency of 93.1% at a current density of 20C. After 2000 cycles, it can still reach a discharge capacity of 122 mA h g⁻¹, and with a capacity retention of 82.7%. This superior high rate performance indicates that it has potential for application as cathode material in high-power lithium-ion batteries.

Fig. 8 (a) The 1st and 2000th charge/discharge profiles and (b) cycle performance and coulombic efficiency of C-LVP/(GNS+CNT) electrode at 20C in 3.0–4.8 V at room temperature (25 °C).

To reveal the reason of the excellent electrochemical performance for GNS and CNT co-modified C-LVP composite, EIS test is carried out after three full cycles at 0.2C. As shown in Fig. 9, all the Nyquist plots of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) are comprised of a depressed semicircle at the high frequency region and a straight line at the low frequency region. It should be mentioned that the semicircle at high frequency region is assigned to the charge-transfer resistance (R_ct) at the electrode/electrolyte interface, while the inclined line at low frequency region corresponds to the Warburg impedance which is related to the Li-ion diffusion process within the electrode.⁶¹ As expected, the charge-transfer resistance (R_ct) of C-LVP/(GNS+CNT) (101 Ω) is much lower than those in C-LVP (266 Ω) and C-LVP/GNS (153 Ω) samples. Besides, the result is also lower than that of GNS-modified LVP electrode in the previously reports.⁴⁰ The significant decrease of charge-transfer resistance (R_ct) of C-LVP/(GNS+CNT) in comparison to that of C-LVP and C-LVP/GNS indicates that the incorporation of GNS and CNT mixed conductive network greatly facilitates the transfer of charge in the composite, which could improve the electrons and Li-ions transport on the surface of LVP particles.

Fig. 9 The Nyquist plots of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) electrodes after three full cycles at 0.2C, respectively.

In order to investigate the electrochemical behavior of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) electrodes, CV curves was carried out in the voltage range of 3.0–4.8 V at a scan rate of 0.1 mV s⁻¹, and the results are shown in Fig. 10. It can be seen that the four peaks in the charge procedure and three peaks in the discharge procedure are present in all of the curves, indicating that all three Li-ions are mobile during the charge/discharge process. Furthermore, after the GNS and CNT co-doping, the anodic peaks shift toward higher potential and the cathodic peaks shift toward lower potential, suggesting that the polarization between each redox has been alleviated and the extraction and insertion process of Li-ions become easier.⁶² Thus, it can be found that the formed crosslinked three-dimensional (3D) structure between GNS and CNT is helpful to improve the reversibility and electrochemical performance of LVP electrode.

Fig. 10 Cyclic voltammetry curves of C-LVP, C-LVP/GNS and C-LVP/(GNS+CNT) electrodes at a scan rate of 0.1 mV s⁻¹ in the voltage range of 3.0–4.8 V.

Conclusions

In summary, we report a new idea in design and preparation of electrode materials for lithium-ion batteries. In this study, the GNS and CNT co-modified C-LVP nanocomposite has been prepared via a hydrothermal-assisted sol–gel method for the first time. Instrumental analyses including XRD, SEM, TEM and Raman spectrum reveal that the nanosized composite consists of a single phase monoclinic C-LVP particles and a three-dimensional (3D) interpenetrating network structure. The added GNS and CNT can greatly improve the electronic conductivity and Li-ions diffusion ability of C-LVP sample. As a result, it shows excellent C-rate performance, delivers capacities of 190.3, 187.9, 185, 181.5, 175.3, 166.2 and 154.6 mA h g⁻¹ at 0.1, 0.2, 0.5, 1, 2, 5 and 10C in the voltage range of 3.0–4.8 V, respectively. Even at 20C, it still shows a discharge capacity of 147.5 mA h g⁻¹. Moreover, the cycling performance is also improved at high voltage. The capacity retention is 82.7% at 20C after 2000 cycles. The superior electrochemical performance of C-LVP/(GNS+CNT) composite indicates that this approach may also be applied to other cathode materials for rechargeable lithium-ion batteries.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 41572245) and Open Project Program of National Engineering Laboratory for Technology of Geological Disaster Prevention in Land Transportation.

Notes and references

1. M. Armand and J. M. Tarascon, Nature, 2008, 451, 652.
2. L. F. Cui, R. Ruffo, C. K. Chan, H. Peng and Y. Cui, Nano Lett., 2009, 9, 491.
3. J. J. Wang and X. L. Sun, Energy Environ. Sci., 2012, 5, 5163.
4. J. B. Goodenough and K. S. Park, J. Am. Chem. Soc., 2013, 135, 1167.
5. P. He, H. R. Wang, L. Qi and T. Osaka, J. Power Sources, 2006, 158, 529.
6. T. Y. S. Panca Putra, M. Yonemura, S. Torii, T. Ishigaki and T. Kamiyama, Solid State Ionics, 2014, 262, 83.
7. Y. X. Gu, Z. L. Tang, Y. Deng and L. Wang, Electrochim. Acta, 2013, 94, 165.
8. S. Patoux and M. M. Doeff, Electrochem. Commun., 2004, 6, 767.
9. A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc., 1997, 144, 1188.
10. B. H. Rong, Y. W. Lu, X. W. Liu, Q. L. Chen, K. Tang, H. Z. Yang, X. Y. Wu, F. Shen, Y. B. Chen, Y. F. Tang and Y. F. Chen, Nano Energy, 2014, 6, 173.
11. R. V. Hagen, H. Lorrmann, K. C. Moller and S. Mathur, Adv. Energy Mater., 2012, 2, 553.
12. H. Huang, S. C. Yin, T. Kerr, N. Taylor and L. F. Nazar, Adv. Mater., 2002, 14, 1525.
13. M. Y. Saidi, J. Barker, H. Huang, J. L. Sowyer and G. Adamson, J. Power Sources, 2003, 119, 266.
14. M. S. Whittingham, Chem. Rev., 2004, 104, 4271.
15. Q. Q. Chen, J. M. Wang, Z. Tang, W. C. He, H. B. Shao and J. Q. Zhang, Electrochim. Acta, 2007, 52, 5251.
16. S. C. Yin, H. Grondey, P. Strobel, H. Huang and L. F. Nazar, J. Am. Chem. Soc., 2003, 125, 326.
17. S. Patoux, C. Wurm, M. Morcrette, G. Rousse and C. Masquelier, J. Power Sources, 2003, 119–121, 278.
18. S. C. Yin, H. Grondey, P. Strobel, M. Anne and L. F. Nazar, J. Am. Chem. Soc., 2003, 125, 10402.
19. X. Rui, N. Ding, J. Liu, C. Li and C. Chen, Electrochim. Acta, 2010, 55, 2384.
20. A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon and W. V. Schalkwijk, Nat. Mater., 2005, 4, 366.
21. X. J. Zhang, W. H. Shi, J. X. Zhu, W. Y. Zhao, J. Ma, S. Mhaisalkar, T. L. Maria, Y. H. Yang, H. Zhang, H. H. Hng and Q. Y. Yan, Nano Res., 2010, 3, 643.
22. Z. Y. Yang, J. H. Hu, Z. Y. Chen, J. Zhong, N. Y. Gu and N. Zhang, RSC Adv., 2015, 5, 17924.
23. Y. H. Kee, N. Dimov, E. Kobayashi, A. Kitajou and S. Okada, Solid State Ionics, 2015, 272, 138.
24. K. Nathiya, D. Bhuvaneswari, D. Nirmala and N. Kalaiselvi, RSC Adv., 2012, 2, 6885.
25. M. Choi, K. Kang, H. S. Kim, Y. M. Lee and B. S. Jin, RSC Adv., 2015, 5, 4872.
26. A. Xia, J. F. Huang, G. Q. Tan and H. J. Ren, Ceram. Int., 2014, 40, 14845.
27. W. C. Duan, Z. Hu, K. Zhang, F. Y. Cheng, Z. L. Tao and J. Chen, Nanoscale, 2013, 5, 6485.
28. Q. L. Wei, Q. Y. An, D. D. Chen, L. Q. Mai, S. Y. Chen, Y. L. Zhao, M. H. kalele, L. Xu, A. M. Khan and Q. J. Zhang, Nano Lett., 2014, 16, 767.
29. P. X. Xiong, L. X. Zeng, H. Li, C. Zheng and M. D. Wei, RSC Adv., 2015, 5, 57127.
30. X. S. Yang, Y. Y. Wang, Y. J. Zhong, B. H. Zhong and Y. Tang, RSC Adv., 2015, 5, 77637.
31. A. K. Geim, Science, 2009, 324, 1530.
32. S. Dubin, S. Gilje, K. Wang, V. C. Tung, K. Cha, A. S. Hall, J. Farrar, R. Varshneya, Y. Yang and R. B. Kaner, ACS Nano, 2010, 4, 3845.
33. L. Wang, L. C. Zhang, I. Lieberwirth, H. W. Xu and C. H. Chen, Electrochem. Commun., 2010, 12, 52.
34. S. K. Zhong, J. Wang, L. T. Liu, J. Q. Liu and Y. W. Li, Ionics, 2010, 16, 117.
35. X. H. Rui, C. Li and C. H. Chen, Electrochim. Acta, 2009, 54, 3374.
36. J. C. Zheng, X. H. Li, Z. X. Wang, S. S. Niu, D. R. Liu, L. Wu, L. J. Li and H. G. Guo, J. Power Sources, 2010, 195, 2935.
37. J. W. Wang, J. Liu, G. L. Yang, X. F. Zhang, X. D. Yan, X. M. Pan and R. S. Wang, Electrochim. Acta, 2009, 54, 6451.
38. A. K. Rai, T. V. Thi, J. Gim, S. Kim and J. Kim, Ceram. Int., 2015, 41, 389.
39. X. H. Rui, D. H. Sim, K. M. Wong, J. X. Zhu, W. L. Liu, C. Xu, H. T. Tan, N. Xiao, H. H. Hng, T. M. Lim and Q. Y. Yan, J. Power Sources, 2012, 214, 171.
40. B. Cheng, X. D. Zhang, X. H. Ma, J. W. Wen, Y. Yu and C. H. Chen, J. Power Sources, 2014, 265, 104.
41. J. F. Zhu, R. S. Yang and K. L. Wu, Ionics, 2013, 19, 577.
42. Y. Jiang, W. W. Xu, D. D. Chen, Z. Jiao, H. J. Zhang, Q. L. Ma, X. H. Cai, B. Zhao and Y. L. Chu, Electrochim. Acta, 2012, 85, 377.
43. D. N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura and S. Iijima, Nat. Mater., 2006, 5, 987.
44. V. Gupta and N. Miura, J. Power Sources, 2006, 157, 616.
45. T. Jiang, C. Z. Wang, G. Chen, H. Chen, Y. J. Wei and X. Li, Solid State Ionics, 2009, 180, 708.
46. Y. Feng, Mater. Chem. Phys., 2010, 121, 302.
47. Y. Zhang, Y. Lv, L. Z. Wang, A. Q. Zhang, Y. H. Song and G. Y. Li, Synth. Met., 2011, 161, 2170.
48. Z. J. Fan, J. Yan, T. Wei, G. Q. Ning, L. J. Zhi and J. C. Liu, ACS Nano, 2011, 5, 2787.
49. S. Chen, P. Chen and Y. Wang, Nanoscale, 2011, 3, 4323.
50. G. Wu, Y. K. Zhou and Z. P. Shao, Appl. Surf. Sci., 2013, 283, 999.
51. S. Y. Yang, K. H. Chang, H. W. Tien, Y. F. Lee, S. M. Li, Y. S. Wang, J. Y. Wang, C. M. Ma and C. C. Hu, J. Mater. Chem., 2011, 21, 2374.
52. E. J. Yoo, J. Kim, E. Hosono, H. S. Zhou, T. Kudo and I. Honma, Nano Lett., 2008, 8, 2277.
53. L. Zhang, S. Q. Wang, D. D. Cai, P. C. Lian, X. F. Zhu, W. S. Yang and H. H. Wang, Electrochim. Acta, 2013, 91, 108.
54. R. Dominko, M. Bele, M. Gaberscek, M. Remskar, D. Hanzel, S. Pejovnik and J. Jamnik, J. Electrochem. Soc., 2005, 152, A607.
55. X. H. Rui, J. X. Zhu, D. Sim, C. Xu, Y. Zeng, H. H. Hng, T. M. Lim and Q. Y. Yan, Nanoscale, 2011, 3, 4752.
56. R. Y. Zhang, Y. Q. Zhang, K. Zhu, F. Du, Q. Fu, X. Yang, Y. H. Wang, X. F. Bie, G. Chen and Y. J. Wei, ACS Appl. Mater. Interfaces, 2014, 6, 12523.
57. H. Yi, C. Hu, H. Fang, B. Yang, Y. Yao, W. Ma and Y. Dai, Electrochim. Acta, 2011, 56, 4052.
58. Y. S. Chen, D. Zhang, X. F. Bian, X. F. Bie, C. Z. Wang, F. Du, M. S. Jang, G. Chen and Y. J. Wei, Electrochim. Acta, 2012, 59, 95.
59. H. D. Liu, P. Gao, J. H. Fang and G. Yang, Chem. Commun., 2011, 47, 9110.
60. E. J. Yoo, J. Kim, E. Hosono, H. S. Zhou, T. Kudo and I. Honma, Nano Lett., 2008, 8, 2277.
61. Y. K. Zhou, J. Wang, Y. Y. Hu, R. O'Hayre and Z. P. Shao, Chem. Commun., 2010, 46, 7151.
62. J. Yan, W. Yuan, Z. Y. Tang, H. Xie, W. F. Mao and L. Ma, J. Power Sources, 2012, 209, 251.

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