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Rational design and synthesis of Li3V2(PO4)3/C nanocomposites as high-performance cathode for lithium ion batteries Tao Chen, Jiang Zhou, Guozhao Fang, Yan Tang, Xiaoping Tan, Anqiang Pan, and Shuquan Liang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03989 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Sustainable Chemistry & Engineering

Rational design and synthesis of Li3V2(PO4)3/C nanocomposites as high-performance cathodes for lithium-ion batteries Tao Chen,† Jiang Zhou*,†,‡ Guozhao Fang,† Yan Tang,†,‡ Xiaoping Tan*,†,‡ Anqiang Pan,†,‡ and Shuquan Liang†,‡

† School of Materials Science and Engineering, Central South University, Changsha 410083, P. R. China ‡ Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, P. R. China Corresponding Author: E-mail: [email protected]; [email protected]

KEYWORDS: Li3V2(PO4)3; carbon-based composites; long-cycle-life; broad temperature adaptability; lithium-ion batteries

ABSTRACT. Synthesis of cathode materials with high rates and long cycle life is critical to meet the demands of high-power lithium-ion battery applications. Batteries that can work at extreme low temperature conditions are also urgently needed. Herein, we report the rational design and synthesis of a novel lithium vanadium phosphate/carbon (Li3V2(PO4)3/C) nanocomposite with a thin-layer amorphous carbon shell (~4 nm) coating and a mesoporous KB carbon matrix. As expected, the Li3V2(PO4)3/C nanocomposite demonstrates an excellent high-rate and long-term cycling performance as well as broad temperature adaptability. Even at a high rate of 16C, a high capacity of 102 mA h g-1 can be achieved. Importantly, the capacity retention is 99% after 1000 cycles and 94% 1

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after 2000 cycles when cycled at a rate of 6 C at room temperature in a voltage range of 3-4.3 V. Importantly, this nanocomposite can cycle at -30°C for 500 cycles with a stable capacity. The excellent rate capability, stable cycling performance, and broad temperature adaptability suggest that the Li3V2(PO4)3/C nanocomposite is a promising cathode for practical applications.

INTRODUCTION Rechargeable lithium-ion batteries (LIBs) are expected to be promising power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs) because of their advantages, which include a high energy density, long lifespan and broad temperature adaptability.1-4 From the viewpoint of electrode materials, lithium transition metal phosphates are the most desirable cathodes for high-power LIBs,5, 6 and actually, the LiFePO4 has become a commercial cathode in the past few years. Interestingly, as a typical phosphate compounds, Li3V2(PO4)3 (LVP) has a relatively higher theoretical capacity, higher average operational voltage plateau and higher Li-ion diffusion coefficient than LiFePO4, making it a highly prospective cathode for high-power LIBs.7-11 In addition, LVP also has the advantages of a thermodynamically stable structure and abundant resources. However, the LVP cathode also suffers from poor high-rate performance and long-term cycling stability because of its intrinsic low electronic conductivity and structural degradation during the Li+ intercalation/de-intercalation process,12, 13 which limits its applications. Previous studies have suggested that synthesis of a Li3V2(PO4)3/C (LVP/C) composite may be an effective way to alleviate the poor performance. Additionally, the performance of LVP/C is greatly affected by the carbon resource and coating quality. Surface carbon coating may greatly improve the 2


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electrical conductivity with enhanced high-rate performance and effectively alleviate the issue of structural degradation during cycling to promote its long-term cycling stability.14 In the previous literature, by coating LVP with a thin layer of amorphous carbon, the LVP/C composites demonstrate enhanced electrochemical performance.5,

14-16

In our previous study, LVP particles can be well

embedded in the KB carbon matrix to produce high-power discharge rates.17 However, surface carbon-coated LVP embedded in a carbon network is seldom reported, and it may greatly improve the electrochemical performance of the LVP cathode. In this work, we report a one-pot method for the rational synthesis of a novel Li3V2(PO4)3/C nanocomposite using both sucrose and KB carbon as carbon sources. Importantly, Li3V2(PO4)3 nanoparticles with a thin layer (~4 nm) of an amorphous carbon shell coating are embedded in the KB carbon matrix. This unique structure has some advantages as follows: 1) the high electrical conductivity provided by the amorphous carbon shell and KB carbon matrix, 2) avoidance of structural degradation during cycling from carbon shell, and 3) rapid ion transport provided by the mesoporous KB carbon matrix. As expected, LVP/C nanocomposite demonstrates the excellent electrochemical performance including a high-rate capability, long-term cycling stability and low-temperature performance. Importantly, the nanocomposite exhibits superior long-term cycling stability with a capacity retention of 94% over 2000 cycles at a high rate of 6 C in the voltage range of 3-4.3 V. EXPERIMENTAL SECTION Li3V2(PO)3/C nanocomposites were prepared using analytically pure chemicals as raw materials, including vanadium pentoxide, oxalic acid, lithium acetate dehydrate, NH4H2PO4, sucrose and KB 3



carbon. In a typical procedure, first, 2.5 g of V2O5, 5.1986 g of C2H2O4·2H2O, 4.2069 g of C2H7LiO4, 4.7430 g of NH4H2PO4 and 2.8988 g of sucrose were put into a bottle containing 80 ml of de-ionized water with magnetic stirring at 80°C. After 2 h, the color of the solution becomes transparent blue. Next, the transparent blue solution was transfer to a beaker follow by the addition of 0.8006 g of KB carbon. The mixture was then ultrasonicated for 1 h to ensure a homogeneous dispersion. Thereafter, the mixed solution was heated to 80°C under active stirring to evaporate the water for several hours until the homogenous slurry was formed. Finally, the slurry was dried overnight in an oven at 80°C to get the solid mixtures. After sufficient grinding, the as-obtained solid mixture was then compressed into tablets followed by heat treatments at 350°C for 4 h and 800°C for 8 h under a flowing gas composed of 95% Ar and 5% H2 in a tube furnace to obtain the Li3V2(PO4)3/C nanocomposite. X-ray diffraction patterns were taken on a Rigaku D/max 2500 diffractometer with Cu Kα radiation. The morphology of LVP/C was observed by scanning electron microscopy (SEM, FEI Sirion200). Then, the more sophisticated morphology and crystal structure characteristics for LVP/C were obtained from transmission electron microscopy (TEM, JEOL JEM-2100F). The Raman spectra were obtained within the 1000-2000 cm-1 region by a spectrometer (Raman, LabRAM HR800) with a back-illuminated charge-coupled detector (CCD) attachment. The carbon content of LVP/C was tested by a carbon-sulfur analyzer (CS-444). The electrochemical properties were obtained by the assembly of coin cells (2025-type coin cell). To prepare the cathode, the N-methyl-2-pyrrolidone (NMP) solution were dispersed into mixtures of samples and polyvinylidene fluoride (PVDF) binder in a weight ratio of 90:10 to make the slurry, and acetylene black was not necessary. Then, the slurry was coated on an aluminum foil and dried in a 4


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vacuum oven at 110°C for 10 h for use as a cathode with a Li metal anode. The Li/LVP/C cells were assembled in a glove box (Mbraun, Germany) filled with ultrahigh purity argon using polypropylene membrane as separator and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate(EC/DMC) (1:1 v/v) as the electrolyte. The cyclic voltammetry (CV) curves were obtained on a CHI604E (CH Instrument electrochemical workstation) apparatus at a scan rate of 0.1 mV s-1, while the galvanostatic cycling experiments were performed on a multichannel battery tester (Land CT 2001A) in different voltage ranges (vs. Li/Li+). In this work, although we used two carbon sources, namely, sucrose and KB carbon, the carbon content of LVP/C was tested to be 19.6 wt.% by the carbon-sulfur analyzer (CS-444). The specific capacity and current density were calculated based on the weight of the LVP material only. The loading of the LVP/C cathode material for the coin cell test was approximately 1-2 mg cm-2.

RESULTS AND DISCUSSIONS The synthetic procedure of the LVP/C nanocomposite is effective and environmentally friendly. First, sucrose is dissolved ensuring a homogeneous dispersed in the precursor solution containing the elements of Li, V and P. Then, KB carbon is added to the solution with ultrasonication and active stirring to obtain a homogenous slurry. High-temperature calcination with a protective atmosphere of 95% Ar and 5% H2 results in the LVP/C nanocomposite. The sucrose may form an amorphous carbon shell coating on the LVP nanoparticles and then embed in the KB carbon network. The phase purity of the novel LVP/C nanocomposite is characterized by X-ray diffraction (XRD), and the result is shown in Fig. 1a, clearly indicating the formation of a highly crystalline phase after calcination. As 5



seen at Fig. 1a, the main diffraction peaks are indexed to the monoclinic phase of Li3V2(PO4)3 with the P21/n space group.15-18 No crystalline carbon diffraction peak are detected, indicating the amorphous carbon species in the LVP/C composite, which is consistent with previous reports.15, 17 Fig. 1b shows the Raman spectra of the LVP/C nanocomposite. Two characteristic bands of carbonaceous materials located at ~1354 cm-1 (D-band, disordered carbon) and ~1592 cm-1 (G-band, graphitic carbon) are clearly observed.5, 19 Moreover, the ID/IG value was calculated to be ~0.97, suggesting a relatively high degree of graphitization that may highly improve the electronic conductivity.20

Fig. 1 (a) XRD pattern and (b) Raman spectra of the LVP/C nanocomposite.

The morphology of the LVP/C nanocomposite is investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the sample displays a loose porous morphology, which is good for electrolyte penetration. The higher magnification SEM image shown in Fig. 2b reveals the LVP particles wrapped by the KB carbon matrix. The TEM image shown in Fig. 2c clearly demonstrates that the LVP nanoparticles are harmoniously distributed among the rag-like KB networks. In addition, there are two different 6


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particle sizes of the LVP particles being observed. Many small particles approximately 50 nm (as indicated by the black arrows in Fig. 2c) are embedded in the carbon matrix. The small particle size is due to the inhibition of particle aggregation by the KB carbon matrix. Some large particles with a size of ~400 nm formed without the inhibition of KB carbon.17 Interestingly, the large LVP particles are also surrounded by the KB carbon, as shown in the inset Fig. 2d. The high-resolution transmission electron microscopy (HRTEM) image (Fig. 2d) shows a clear lattice fringe with a d-spacing of 0.374 nm, which is in good agreement with the (121) interplanar distance of monoclinic Li3V2(PO4)3. Importantly, each particle is coated with a thin, uniform layer of an amorphous carbon shell with a thickness of ~4 nm. The amorphous carbon shell may be due to the high temperature carbonization of sucrose. The TEM results demonstrate that the novel LVP/C nanocomposite constructed by amorphous carbon-coated LVP nanoparticles embedded in the KB carbon matrix has been successfully prepared. Generally, the LVP/C composites reported by the previous studies usually contain only one kind of carbon. The novel composite structure obtained in this work may no doubt benefit from the advantages of the amorphous carbon coating and the KB carbon matrix with enhanced electrochemical performance. Moreover, in order to investigate the specific surface area of LVP/C, the N2 adsorption/desorption isotherm (Fig. 3a) is obtained. The curve displays a structural type IV characteristic, indicating the porous property of the LVP/C. According to the multipoint BET method, a specific surface area of 27.9 m2 g-1 was achieved for LVP/C composites. The BJH curve (Fig. 3b) demonstrates a narrow pore diameter distribution of approximately 3.9 nm.

7



Fig. 2 (a, b) SEM images, (c) TEM image and (d) HRTEM image of LVP/C nanocomposite. The inset (d) shows a TEM image of an individual particle.

Fig. 3 (a) N2 adsorption/desorption isotherm and (b) corresponding BET pore-size distribution curves of the LVP/C nanocomposite. 8


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For verification, electrochemical measurements (Fig. 4) of the LVP/C nanocomposites were performed to investigate the electrochemical performance. Fig. 4a shows the cyclic voltammetry (CV) results for the novel LVP/C nanocomposite. The voltammograms were measured at a scan rate of 0.1 mV s-1 in the potential range from 4.3 to 3.0 V vs. Li/ Li+ at ~28°C. Apparently, three pairs of anodic and cathodic peaks corresponding to the V3+/V4+ redox couple were observed for the electrode. The two oxidation peaks approximately 3.62 and 3.70 V correspond to the extraction of the first Li+ ion in two steps, with the existence of an ordered phase Li2.5V2(PO4)3 at a mixed V3+/V4+ state 21

. The second Li+ ion was extracted through a single step approximately 4.11 V, with the complete

oxidation of V3+ to V4+. The reduction peaks located at ~3.56, 3.63 and 4.0 V are ascribed to the reversible insertion of two Li+ ions accompanied by the phase transition of LixV2(PO4)3 from x=1.0 to 1.5, 2.0, and 3.0.22 The well-defined anodic and cathodic peaks with smaller value of potential interval suggest good electrochemical reaction reversibility of the LVP/C nanocomposite

23

. In

addition, the CV curve of the 1st cycle is almost overlapped by the 2nd cycle, which indicates good reversibility of LVP/C electrode.

9



Fig. 4 (a) Cyclic voltammetry curves and (b) selected charge/discharge curves at different rates of the LVP/C electrodes. (d) The selected charge/discharge curves at 6 C, and (c, e) the rate performance and long-term cycling performance of LVP/C electrodes.

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We further investigated the battery performance of the LVP/C nanocomposite by carrying out galvanostatic charge/discharge measurements in the voltage range of 3.0-4.3 V vs Li/ Li+. Fig. 4b displays the selected charge/discharge curves of LVP/C electrode at different rates. It can be clearly seen that the 1 C and 2 C rates exhibit three distinct charge plateaus and correspondingly three discharge plateaus, which are consistent with the CV results. Fig. 4c shows the rate performance of the LVP/C nanocomposite. In the voltage range of 3-4.3 V, 1 C is equal to 133 mA h g-1. Interestingly, the LVP/C electrode exhibits high discharge capacities of 124, 125, 123 and 118 mA h g-1 at the rates of 1, 2, 4 and 8 C, respectively. The capacities at 2 C are slightly higher than those at 1C, which may be due to the activation process of the electrode in the initial discharge/charge states at a low current density. Even at a high rate of 16 C, a relatively high capacity of 102 mA h g-1 can also be achieved. Importantly, a high capacity of 122 mA h g-1 returns at the 4 C rate, with no obvious capacity fading for 100 cycles. The above result demonstrates that the LVP/C electrode has an excellent rate performance and good capacity retention at different discharge rates. The long-term cycling stability of the LVP/C nanocomposite is investigated at a high rate of 6C, and the result is shown in Fig. 4e. Generally, the electrodes are initially charged/discharged at low current densities to activate the electrode. In this test, all electrodes are charged/discharged at 200 mA g-1 for 6 cycles, and then, the rate is increased to 6 C. When the current density is increased to 6 C in the 7th cycle, the electrode exhibits a high capacity of 110 mA h g-1. In addition, the capacity retention is as high as 99% for 1000 discharge/charge cycles. Even after 2000 cycles, the electrode can also retain its capacity of 94 mA h g-1, corresponding to a capacity retention of 94%. Moreover, 11



the shape of the charge/discharge curves (Fig. 4d) for the LVP/C electrode at 6 C is almost the same, indicating the high reversibility of this electrode at a high rate. Despite the remarkable capacity, high rate capability, and long cycle life, the broad temperature adaptability is a critical parameter for practical LIBs, especially for northern regions in winter. The cycling performance of LVP/C tested at the different temperatures of 0 and -30°C is shown in Fig. 5a, and the current density is 4C. The initial 100 cycles are tested at 0°C, and LVP/C demonstrates a stable capacity of ~117 mA h g-1. Amazingly, the electrode can cycle at an ultralow testing temperature of -30°C for 500 cycles, with a stable capacity of ~91 mA h g-1, corresponding to a capacity retention of 78% at 0°C. More importantly, its coulombic efficiency remains steady at ~99%. The rate and cycling performance of LVP/C tested at 50°C is shown in Fig. 5b, demonstrating a good rate capability and long-term cycling performance. The initial discharge capacity is 130 mA h g-1 at 0.5 C. After the rate test, the electrode shows a stable cycling performance at 4 C for 400 cycles. In conclusion, the LVP/C nanocomposites demonstrate excellent electrochemical performance at broad temperature test conditions, which is useful for practical applications.

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Fig. 5 The cycling performance of LVP/C tested at different temperatures of (a) 0 and -30°C, (b) 50°C.

The electrochemical performance of LVP/C electrodes over a broad voltage range has also been evaluated. In the voltage range of 3-4.8 V and 3-4.5 V, 1 C is equal to 160 mA h g-1 in this work. The cycling performance of LVP/C tested at 4 C in a voltage range of 3-4.8 V is shown in Fig. 6a. Although the high specific discharge capacity of 158 mA h g-1 can be obtained, it quickly decreases to 125 mA h g-1 after 6 cycles, which may be due to the issues of severe electrolyte decomposition or vanadium dissolution at a high voltage of 4.8 V.24 However, the discharge capacity becomes relatively stable in the subsequent 500 cycles. The electrode demonstrates an excellent rate 13



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capability and cycling stability in the voltage range of 3-4.5 V, as shown in Fig. 6b. The discharge capacity is 156 mA h g-1 at 0.5 C. Even cycled at 16 C, the capacity was maintained as 83 mA h g-1. When the current density is returned to a 4 C rate, the electrode showed a very stable capacity even after 500 cycles. Regardless, the electrochemical performance of the LVP/C electrode over a broad voltage range is worse than that in a narrow voltage range of 3-4.3 V, which may be effectively improved by surface oxide modifications or doping/substitution with ions.

Fig. 6 (a) Long-term cycling performance at 4 C and (b) rate performance of Li3V2(PO4)3/C in the voltage range of 3-4.8 V and 3-4.5 V, respectively.

As mentioned above, the LVP/C electrode demonstrates excellent electrochemical performance, including high power discharge rates and long-term cycling stability at broad temperatures. Our results are superior to many reported results for LVP/C composites.17,

25-28

The superior

electrochemical performance may be due to the following three aspects: 1) the carbon coating and the good conductive KB carbon matrix may highly enhance the electrical conductivity of the composite electrode;29 2) the buffered protective carbon shell can effectively avoid structural degradation during cycling;30 and 3) the KB carbon network enables rapid ion transportation.31, 32 Our work demonstrates 14


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that the novel LVP/C nanocomposite can be a promising cathode for use in high-power lithium-ion battery applications. In addition, our work provides a route to improve the electrochemical performance of electrodes for energy applications through the synthesis of composite materials constructed by surface carbon coating and mesoporous carbon network.

CONCLUSIONS In conclusion, we report the rational design and synthesis of a novel Li3V2(PO4)3/C nanocomposite, with a thin layer carbon shell coating Li3V2(PO4)3 was embedded in the KB carbon matrix. The Li3V2(PO4)3/C nanocomposite exhibits an excellent rate capacity (102 mA h g-1 at 16 C) and long-term cycling stability (the capacity retention is 99% after 1000 cycles and 94% after 2000 cycles when cycled at a rate of 6 C). Most importantly, a high capacity of ~91 mA h g-1 can be reached at an ultralow testing temperature of -30°C. The excellent rate performance and low-temperature performance are due to the novel Li3V2(PO4)3/C nanocomposite structure. Both the carbon-coating and KB carbon matrix can greatly improve the conductivity and lithium diffusion ability of Li3V2(PO4)3. Carbon-coating can protect the structure from degradation. This work provides a new idea to develop high-performance electrodes for energy applications by constructing carbon-based composite materials with a carbon coating and mesoporous carbon network.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; 15



[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant no. 51374255 and 51302323), Innovation-Driven Project of Central South University (No. 2018CX004) and the Fundamental Research Funds for the Central Universities of Central South University.

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For Table of Contents Use Only

A novel Li3V2(PO4)3/C nanocomposite is synthesized and demonstrates an excellent high-rate capability, long-term cycling stability, and broad temperature adaptability as a cathode for lithium-ion batteries.

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A novel Li3V2(PO4)3/C nanocomposite is designed and synthesized with a thin layer amorphous carbon shell (~4 nm) coating and a mesoporous KB carbon matrix. Expectedly, the Li3V2(PO4)3/C nanocomposite demonstrates excellent high-rate capability, long-term cycling stability, as well as broad temperature adaptability. 84x40mm (300 x 300 DPI)


C nanocomposites as

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