C composite as a novel anode material for lithium-ion batteries

C composite as a novel anode material for lithium-ion batteries

Journal of Power Sources 441 (2019) 227180 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/loc...

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Journal of Power Sources 441 (2019) 227180

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Short communication

CrPO4/C composite as a novel anode material for lithium-ion batteries Lizhen Hu a, Shuai Zheng a, Siqi Cheng a, Zhuo Chen a, Bin Huang a, Qingquan Liu b, Shunhua Xiao a, Jianwen Yang a, Chen Quanqia, * a

Guangxi Key Laboratory of Electrochemical and Magneto-chemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin, 541004, China b Hunan Provincial Key Laboratory of Advanced Materials for New Energy Storage and Conversion, Hunan University of Science and Technology, Xiangtan, 411201, China

H I G H L I G H T S

� CrPO4 was prepared by hydrothermal reaction and high-temperature calcination in air. � CrPO4/C was synthesized by calcination of mixture of CrPO4 and glucose in inert gas. � Both CrPO4 and CrPO4/C were first investigated as anodes for lithium-ion batteries. � CrPO4 exhibits initial charge capacity of 342.8 mAh g 1 at 0.2C (109.4 mA g 1). � CrPO4/C shows initial charge capacity of 616.5 mAh g 1 at 2C and better cyclability. A R T I C L E I N F O

A B S T R A C T

Keywords: Lithium-ion batteries Anode Phosphate CrPO4 Polyanion

The carbon coated CrPO4 (CrPO4/C) is prepared by high-temperature calcination of mixture glucose and CrPO4 under inert atmosphere, in which CrPO4 is synthesized by a combination of hydrothermal reaction and hightemperature sintering in air. The CrPO4 and CrPO4/C are first investigated as anode materials for lithium-ion batteries in this work, and CrPO4/C displays higher capacity, better cyclability and rate capability than CrPO4. In the voltage range of 0.01–3.0 V (vs. Liþ/Li), CrPO4 presents initial charge capacity of 342.8 mAh g 1 at 0.2C (109.4 mA g 1), while CrPO4/C shows higher initial charge capacity of 686.7 mAh g 1 at 0.2C and reversible capacity of 397.7 mAh g 1 after 60 cycles. Even at high current rate of 1094 mA g 1, CrPO4/C exhibits high initial charge capacity of 616.5 mAh g 1 and reversible capacity of 272 mAh g 1 after 100 cycles, much higher than initial charge capacity of 266.7 mAh g 1 and reversible capacity of 78 mAh g 1 after 100 cycles for CrPO4 under the same galvanostatically charged/discharge conditions. The results demonstrate that CrPO4-based composite will be a promising anode material for LIBs after further optimization of synthesis and components.

1. Introduction

Therefore, it is urgent and crucial to boost electrochemical performance of the above-mentioned anodes and search for new anode materials for LIBs. Recently polyanionic compounds containing PO34 group such as VPO4 [31–36] and VOPO4 [37] have been explored as anodes for LIBs. Based on the conversion reactions, VPO4 and VOPO4 anodes present theoretic capacity of 551 and 827 mAh g 1, respectively, and better cyclability than that of other conversion-type anode materials. The relatively better cyclability of these two phosphates is attributed to the larger polyanion PO34 , which can provide three-dimension transport channel for rapid diffusion of Liþ, make the framework steady, and abate volume changes during discharge and charge processes. The

To meet the requirements of high-energy density lithium-ion batte­ ries (LIBs) based on the advanced anode materials, a great deal of efforts have been made to develop higher capacity of alternatives to the com­ mercial graphite anode [1], which has lower theoretic capacity of only 372 mAh g 1 [2]. The metals oxides [3–9], alloys [10–14], metals [15–18], metal sulfides [19–24], non-metals [25–28] and metal nitrides [29,30] have been intensively reported as anode materials for LIBs. However, it is still a big challenge to improve cyclability of the afore­ mentioned anodes because they undergo large volume change and possible pulverization during discharge and charge processes. * Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Chen).

https://doi.org/10.1016/j.jpowsour.2019.227180 Received 23 April 2019; Received in revised form 16 August 2019; Accepted 18 September 2019 Available online 17 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

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electrochemical performance of VPO4 is significantly improved by introduction of graphene or composite of graphene and carbon [32,34], implying that transition metal phosphates will turn to be a new prom­ ising type of anode materials after their electronic conductivity is greatly improved. Although metal phosphates used as anode materials exhibit the above-mentioned merits, other transition metal phosphates have been not reported yet. Herein, we report a new phosphate anode material CrPO4, which is extraordinary stable and commonly used as pigment and catalyst for synthesis of organic compounds. In this work, CrPO4 was prepared by a simple hydrothermal process and the followed high-temperature calci­ nation in air, and the carbon coated CrPO4 (CrPO4/C) was synthesized by sintering the mixture of CrPO4 and glucose at high temperature in inert gas atmosphere. The electrochemical performance of CrPO4 and CrPO4/C used as anodes in LIBs was evaluated by cyclic voltammetry and galvanostatic cycling at various current rates.

3. Results and discussion Compared with the green CrPO4 sample, the CrPO4/C composite is black due to existence of carbon resulting from the pyrolysis of glucose in inert atmosphere at high temperature, and the carbon content determined by carbon–sulfur analyzer is about 5.1 wt%. The XRD pat­ terns of CrPO4 and CrPO4/C are shown in Fig. 1a, and all the diffraction peaks are indexed by space group cmcm (No. 63), in consistence with CrPO4 (PDF: 09-0384). The cell parameters for CrPO4 and CrPO4/C are a ¼ 0.5172 nm, b ¼ 0.7757 nm, c ¼ 0.6132 nm and a ¼ 0.5178 nm, b ¼ 0.7764 nm, c ¼ 0.6129 nm, respectively, and the similarity in cell parameters of CrPO4 and CrPO4/C indicates that the introduction of carbon does not play significant impacts on the cell parameters. It is noted that no carbon diffraction peaks appear in the XRD patterns of CrPO4/C, which implies that carbon may be amorphous in the CrPO4/C composite. To confirm the characteristic of carbon in the CrPO4/C composite, Raman spectra of CrPO4 and CrPO4/C presented in Fig. 1b are investigated and the characteristic Raman peaks of CrPO4 are observed in both two samples. However, compared with pristine CrPO4, two additional peaks at about 1338.5 and 1584.6 cm 1 occur in the Raman spectra of CrPO4/C composite, which could be ascribed to characteristic D-band and G-band of carbon, respectively. The low in­ tensity ratio (0.89) of D-band and G-band (ID/IG) indicates that carbon in CrPO4/C composite has the properties of highly graphitic carbon, which is helpful to improve the electronic conductivity of CrPO4/C composite, resulting in improved electrochemical performance of CrPO4. The SEM images of CrPO4 and CrPO4/C depicted in Fig. 1c and d, respectively, show that the particle size of both CrPO4 and CrPO4/C is smaller than 500 nm. Compared with the smooth surface of CrPO4, the surface of CrPO4/C is coarser, indicating that CrPO4 may be covered by amorphous carbon in the CrPO4/C. It also can be seen that the amor­ phous carbon can bridge the CrPO4 particles, which is beneficial for the rapid transfer of charger at the surface of CrPO4. The SEM image and the corresponding energy dispersive spectroscopy (EDS) element mappings of CrPO4/C are displayed in Fig. 2a, in which elements Cr, P and O are of homogeneous distribution and element C exhibits non-uniform distri­ bution in the CrPO4/C, further revealing that carbon is not incorporated into CrPO4 crystal, which implies that CrPO4 particles may be evenly coated by carbon. Fig. 2b and c shows the typical TEM images of CrPO4/C, in which CrPO4 particles are of diameters smaller than 500 nm and are coated by the thin carbon layer, and a batch of CrPO4 particles are connected by the amorphous carbon, in consistence with SEM observation. As shown in Fig. 2d and e, the lattice fringes of CrPO4 display interplanar spacings of 0.1638, 0.3276, 0.2498 and 0.1764 nm in the particles, corresponding respectively to those of the planes (042), (021), (112) and (232) of orthorhombic CrPO4. As an analogue of conversion-type anode material VPO4 [31], CrPO4 may have similar electrochemical reaction mechanism. To understand the discharge/charge mechanism of CrPO4, the ex-situ XRD measure­ ments of fresh, 1st fully discharged and 1st fully charged CrPO4 elec­ trodes were carried out, and the corresponding results are shown in Fig. s1 (in supplementary material). The XRD results demonstrate that Li3PO4 and Cr occur on the electrode when CrPO4 is initially fully dis­ charged to 0.01 V. Subsequently, the 1st fully discharged CrPO4 elec­ trode is charged to 3.0 V and then yields CrPO4. Combined with XRD results and the similar conversion mechanism of VPO4 [31], the con­ version reaction of CrPO4 may be described by the following eqns. (1) and (2).

2. Experimental 2.1. Synthesis of CrPO4 and CrPO4/C The chromium phosphate (CrPO4) was prepared by a facile hydro­ thermal reaction and a followed high temperature calcination process. Typically, 2.2306 g of NH4H2PO4 and 8.0025 g of Cr(NO3)3⋅9H2O were firstly dissolved by 100 mL of distilled water, and the resulted mixture solution was heated 80 � C with stirring in a thermostatic bath for 2 h. Then, the resulted solution (~80 mL) was transferred to a Teflon-lined stainless steel autoclave with capacity of 100 mL, maintained at 180 � C for 12 h, cooled to room temperature, and then the resulted so­ lution evaporated at 80 � C in a thermostatic bath and dried at 120 � C vacuum oven to form a green powder. Finally, the powder was sintered in air at 350 � C for 6 h and 850 � C for 6 h to obtain CrPO4. The CrPO4/C composite was prepared by calcination of the mixture of 1.4697 g of CrPO4 and 0.8090 g of glucose (C6H12O6⋅H2O) under Ar atmosphere at 700 � C for 2 h at a heating rate of 5 � C min 1. 2.2. Physical characterizations The crystal structure of CrPO4 and CrPO4/C samples was investi­ gated by X-ray diffraction (XRD) measurement using an X’Pert3 powder (PANalytical Ltd., Holland) with Cu Ka radiation (λ ¼ 1.54056 Å) in a 2θ range of 10� –80� at the scan rate of 2� min 1. Raman analysis of CrPO4 and CrPO4/C samples was carried out by a Renishaw 1000 spectrometer with an excitation wavelength of 780 nm at a laser power of 20 mW in the range of 0–2500 cm 1. The microstructure and morphology of samples were investigated by scanning electron microscopy (SEM, HITACHI, SU5000) and transmission electron microscopy (TEM, JEOL, JEM-2100F). The carbon content in CrPO4/C was determined by a car­ bon–sulfur analyzer (Mlti EA2000). 2.3. Electrochemical measurements The electrochemical performance of CrPO4 and CrPO4/C samples was evaluated using 2016-type coin cells assembled in an argon-filled glove box. The coin cell is mainly composed of working electrode, lithium foil counter electrode, Cellguard2400 film separator and 1 mol/ L LiPF6 electrolyte in EC/DMC (1:1 by volume). The working electrode was prepared by pasting the mixture slurry onto copper foil, in which the slurry contains CrPO4 or CrPO4/C, acetylene black and PVDF in a weight ratio of 8:1:1 in N-methylpyrrolidone (NMP), and drying the slurry at 120 � C for 12 h in a vacuum oven. The loading of electroactive material on the electrodes is about 2–3 mg cm 2. The cells were galvanostatically discharged/charged at various current rates (1C ¼ 547 mA g 1) in the voltage range of 0.01–3.0 V (vs. Liþ/Li). The specific capacity and cur­ rent density were calculated based on the total weight of CrPO4 or CrPO4/C.

CrPO4 þ 3Liþ þ 3e→Li3 PO4 þ Cr

(1)

Li3 PO4 þ Cr→CrPO4 þ 3Liþ þ 3e

(2)

Fig. 3a and b respectively depict the first three cyclic voltammo­ grams (CVs) of CrPO4 and CrPO4/C electrodes in the potential range of 0.01–3.0 V (vs. Liþ/Li) at a scanning rate of 0.1 mV s 1. The similar 2

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Fig. 1. XRD patterns (a) and Raman spectra (b) of CrPO4 and CrPO4/C; SEM images of CrPO4 (c) and CrPO4/C (d). The insets of (c) and (d) are the corresponding SEM images with low magnification.

shape of CVs in the corresponding cycles of both CrPO4 and CrPO4/C electrodes suggests that the introduction of carbon does not alter the electrochemical reaction mechanism of CrPO4 during discharge and charge processes. In the first CV cycle, an inconspicuous reduction peak of both CrPO4 and CrPO4/C electrodes locates at about 1.2 V, and a sharp reduction peak of CrPO4 and CrPO4/C electrodes situates at around 0.47 and 0.57 V, respectively, which can be attributed to the possible conversion reaction of CrPO4 (eqn. (1)), accompanying decomposition of electrolyte and formation of solid electrolyte interface (SEI) layer, and this phenomenon is similar to that of other anode ma­ terials such as MnO [38], Co3O4 [39,40], Co3S4 [41] and NiO [6]. The initial reduction peak potential of CrPO4/C is higher than that of CrPO4, which results from the lower polarization of CrPO4/C during the discharge process due to existence of carbon with high electronic con­ ductivity. Two broad anodic peaks at about 1.03 and 1.93 V are asso­ ciated with the oxidation of Cr, which may be interpreted by the possible conversion reaction (eqn. (2)). It is noted that the CVs of the second and third cycles of CrPO4 and CrPO4/C are significantly different from the initial CVs, inferring that electrochemical reactions of the first cycle differ from those of the subsequent cycles. The peak current density of the abrupt reduction peak at about 0.5 V significantly decreases and the peak becomes unobvious and a new broad cathodic peak occurs at around 1.06 V after the first CV cycle, indicating that stable SEI layer forms in the first cycle, and decomposition of electrolyte and formation of SEI layer are pro­ nounced factor to result in higher irreversible capacity. The abnormally high overlap of CVs of the second and third cycles demonstrates that both CrPO4 and CrPO4/C may have better cycle performance. As shown in Figs. 3a and b, the peak current densities for the second and third CVs

of CrPO4/C are higher than those of CrPO4, hinting that CrPO4/C ex­ hibits lower polarization and better kinetics performance than CrPO4 during discharge and charge processes, resulting in better electro­ chemical performance of CrPO4/C. The galvanostatic discharge/charge profiles of selected cycles for CrPO4 and CrPO4/C at 0.2C in the voltage range of 0.01–3.0 V (vs. Liþ/ Li) are presented in Fig. 3c and d, respectively. In the initial discharge step, both CrPO4 and CrPO4/C display a long voltage plateau at about 0.66 V for CrPO4 and 0.75 V for CrPO4/C, followed by an inclined curve down to 0.01 V, which is associated with conversion reaction of CrPO4, decomposition of electrolyte and formation of SEI layer [42], in consistent with the result of CV. The long discharge voltage plateau occurring in the first cycle disappears in the subsequent charged/dis­ charged cycles of CrPO4 and CrPO4/C, which is similar to the case of VPO4 [31] and Co3O4 [39], suggesting drastic, lithium-driven, structural and textural modifications [43]. These modifications relate with the formation of Cr, Li3PO4 and stable SEI layer, and decomposition of electrolyte during the first discharge process. The discharge voltage plateau shortens and shifts to higher voltage in the subsequent cycles, which may be linked with possibility that polarization becomes smaller due to formation of ionic conductor Li3PO4 and electronic conductor Cr during the initial cycle, and the change trend in voltage coincides with CV results. As displayed in Fig. 3c, the initial discharge and charge capacities of CrPO4 are 701.9 and 342.8 mAh g 1, respectively, corresponding to low coulombic efficiency of 48.8%, while the coulombic efficiency greatly increases from 83.4% at the second cycle to near 100% for the subse­ quent cycles. The discharge capacity of CrPO4 significantly decreases to 408.4 mAh g 1 at the second cycle, which is still much higher than that 3

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Fig. 2. SEM image and the corresponding element mappings of CrPO4/C (a); TEM images (b–c) and HRTEM images (d–e) of CrPO4/C.

discharge capacity (~300 mAh g 1) of VPO4 at 20 mA g 1 [31] and capacity of 372 mAh g 1 for the graphite. The initial charge capacity of CrPO4 is only about 62.8% of the theoretic capacity, and the charge capacity decreases to 131.1 mAh g 1 after 60 cycles, corresponding to capacity retention of 38.2%. While observed in Fig. 3d, CrPO4/C dis­ plays higher capacities of 1046.7 and 686.7 mAh g 1 for the initial discharge and charge steps, respectively, matching with coulombic ef­ ficiency of 65.6%, which is higher than that of 60.7% for Cr2O3/C [9].

After 60 cycles, the charge capacity of CrPO4/C is reduced to 397.7 mAh g 1, which is still larger than 340.3 mAh g 1 for VPO4/C at 20 mA g 1 after 30 cycles [31], and the capacity retention of CrPO4/C is 19.7% higher than that of CrPO4. It is noted that the initial discharge capacities of both CrPO4 and CrPO4/C are larger than the theoretic capacity of 546 mAh g 1 for CrPO4, which is calculated according to eqn. (1), and the extra capacity and large irreversible capacity can be mainly ascribed to the decomposition of electrolyte, formation of SEI layer and the 4

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Fig. 3. First three cyclic voltammograms of CrPO4 (a) and CrPO4/C (b) in the potential range of 0.01–3.0 V (vs. Liþ/Li) at a scanning rate of 0.1 mV s 1; discharge/ charge profiles of CrPO4 (c) and CrPO4/C (d) at a current rate of 0.2C in the selected cycles; long-term cycle performance of CrPO4 and CrPO4/C at 1C (e) and 2C (f).

interface between carbon and CrPO4. Interestingly, the initial charge capacity of CrPO4/C is higher than theoretic capacity of CrPO4 and carbon, which may be attributed to synergistic effects of CrPO4 and carbon layer, similar to VPO4/C [31], VPO4/(C þ Ag) [36], Co3O4/CNT [39], Co3O4/graphene [40] and Co(OH)2/graphene [44]. The cycle performance of CrPO4 and CrPO4/C at 1C and 2C is respectively shown in Fig. 3e and f, and it can be clearly seen that charge capacity is close to discharge capacity with cycles, corresponding to columbic efficiency of about 100%. It can be found that capacities of

both two samples decrease with current rates increasing due to larger polarization at high current rates. The initial charge capacity of CrPO4 decreases from 275.1 mAh g 1 at 1C to 266.7 mAh g 1 at 2C, which correspond to 80.3% and 77.8% of capacity of CrPO4 at 0.2C, respec­ tively. However, the initial charge capacities of CrPO4/C are 628.4 and 616.5 mAh g 1 at 1C and 2C, respectively, which match well with 91.5% and 89.8% of capacity of CrPO4/C at 0.2C, respectively. The abovementioned comparison suggests that CrPO4/C possesses much better rate capability than CrPO4. It is apparent that the capacity of both CrPO4 5

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and CrPO4/C remarkably decreases before about 30 cycles and then decays sluggishly. The fast capacity lost after the first cycle may result from the abrupt change in textural structure of electrode in the first dozens of cycles, leading to loose contact of Cr and Li3PO4 particles, which is detrimental to the complete conversion of CrPO4 þ 3Liþ þ 3e ↔ Cr þLi3 PO4 and results in the reduction of utilization of CrPO4 during the discharge/charge processes. However, with repeated cycles increasing, the stable texture of electrode forms and the contact of Cr and Li3PO4 does not deteriorate further, which is beneficial to improve the cyclability of CrPO4 and CrPO4/C electrodes in the subsequent cy­ cles. This observed phenomenon is similar to that of VPO4 [31,35,36]. After 100 cycles, CrPO4/C still exhibits higher capacity of 314 and 272 mAh g 1 at 1C and 2C, respectively, while CrPO4 displays lower capacity of 84 and 78 mAh g 1 at 1C and 2C, respectively. The above-mentioned electrochemical comparison between CrPO4 and CrPO4/C reveals that carbon layer plays important roles in the improvement of electro­ chemical performance of CrPO4. The coating carbon can not only improve the electronic conductivity of electroactive materials but also can buffer volume expansion/contract of CrPO4 during discharging and charging processes, and bridges the electroactive CrPO4 particles as well. Additionally, carbon is also an electroactive material and provides capacity, and the interface between carbon and CrPO4 may create more active sites for storage Liþ.

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4. Conclusions The novel carbon coated CrPO4 (CrPO4/C) was simply prepared by calcination of glucose and CrPO4, which was synthesized by a hydro­ thermal reaction and subsequent high-temperature calcination process using Cr(NO3)3⋅9H2O and NH4H2PO4 and as raw materials, in inert at­ mosphere. The CrPO4/C presents higher capacity, better rate capability and cyclability than the pristine CrPO4, and the improvement in elec­ trochemical performance can be ascribed to improved electronic con­ ductivity by carbon layer and the synergetic effects of CrPO4 and carbon. Although cyclability and capacity of CrPO4 are greatly improved by carbon coating, the cycle performance of CrPO4 is still moderate. It is necessary to further enhance the electrochemical performance of CrPO4 by improving the preparation method and optimization of components of CrPO4-based composite. Acknowledgements The authors gratefully acknowledge financial support from the Na­ tional Natural Science Foundation of China (No. 51364007), Science and Technology Major Project of Guangxi (No. AA19046001), and the Natural Science Foundation of Guangxi Province (No. 2013GXNSFAA019304l). Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.227180. References [1] W. Qi, J.G. Shapter, Q. Wu, T. Yin, G. Gao, D. Cui, Nanostructured anode materials for lithium-ion batteries: principle, recent progress and future perspectives, J. Mater. Chem. 5 (2017) 19521–19540. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [3] Y.J. Hong, M.Y. Son, Y.C. Kang, One-Pot Facile Synthesis of Double-Shelled SnO2 Yolk-shell-structured powders by continuous process as anode materials for Li-ion batteries, Adv. Mater. 25 (2013) 2279–2283. [4] J. Lin, Z. Peng, C. Xiang, G. Ruan, Z. Yan, D. Natelson, J.M. Tour, Graphene nanoribbon and nanostructured SnO2 composite anodes for lithium ion batteries, ACS Nano 7 (2013) 6001–6006. [5] M. Dirican, M. Yanilmaz, K. Fu, Y. Lu, H. Kizil, X. Zhang, Carbon-enhanced electrodeposited SnO2/carbon nanofiber composites as anode for lithium-ion batteries, J. Power Sources 264 (2014) 240–247.

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