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Fabrication, structure, electrochemical properties and lithium-ion storage performance of Nd:BiVO4 nanocrystals
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication, structure, electrochemical properties and lithium-ion storage performance of Nd:BiVO4 nanocrystals Ruishi Xie1, Yuanli Li1,∗, Heyan Huang, Li Su, Ling Li, Xiaoqin Pan, Zhiyuan Guo, Fen Luo, Zhicheng Guo∗, Baogang Guo, Fangting Chi∗, Yongjun Ma, Haifeng Liu∗ Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, National Co-Innovation Center for Nuclear Waste Disposal and Environmental Safety, Laboratory for Extreme Conditions Matter Properties, Analytical and Testing Center, Southwest University of Science and Technology, Mianyang, 621010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanostructures Electrochemical properties Lithium ion batteries
Nd:BiVO4 nanocrystals were synthesized by an effective and simple approach. Nd was incorporated into BiVO4 host to enhance electronic conductivity and lithium-ion diffusion kinetics, thus promoting the electrochemical performance. When employed as anode materials in lithium-ion batteries, a good cycling stability and high capacity of ~611 mAh g−1 at 100 mA g−1 were delivered. The work can be extensively employed for fabricating other electrode materials and promoting the electrochemical performance in energy storage correlative domains.
1. Introduction Battery technology is widely recognized as essential for reducing the energy and environmental troubles, since it can be combined with renewable energy and easily utilized in many fields, e.g., renewable energy storage systems, transportation systems and portable electronics [1–3]. In recent years, the booming market for lithium-ion batteries has promptly spread from consumer electronic products to the automotive industry [4–7]. Nevertheless, the finite abundance of lithium-salt brings about the cost increase of lithium-ion batteries and hampers their further development. Furthermore, commercial graphite anode materials possess a low theory capacity, that is not enough to meet the increasing requirement for high power density. Consequently, it is urgent to employ new anode materials for improving the battery technologies in lithium-ion batteries (LIBs) [5,8]. BiVO4 compounds have a framework structure constituted by regular array of angularity-shaped VO4 tetrahedrons and BiO8 dodecahedeons [9,10]. It is a promising anode alternative for LIBs, due to its low poisonousness, high theoretical capacity and abundant elements in nature. Nevertheless, BiVO4 is generally subjected to the aggregation in alloying/dealloying procedure, large volume inflation and low electroconductibility, which brings about discontented rate capability and cycle stability. Great efforts have been made for handling the problems, including designing nano/micro structures, carbon-packing, reducing particle size. These approaches broke down for improving the diffusion coefficient of lithium-ion and intrinsic electronic conductivity. Toward
this end, the combination of nanostructured BiVO4 with incorporating metal ion would be a charming strategy for enhancing the electrochemical performance of BiVO4 LIBs. Here, we demonstrate an effective and simple approach to synthesize Nd doped BiVO4 nanocrystals. Nd was incorporated into BiVO4 host to enhance electronic conductivity and lithium-ion diffusion kinetics, thus promoting the electrochemical performance. A good cycling stability and high capacity of ~611 mAh g−1 at 100 mA g−1 were delivered, which are of tremendous importance for high performance LIBs. The work can be extensively employed for fabricating other metal compounds and promoting the electrochemical performance in energy storage correlative domains. 2. Experimental In a typical experiment, for synthesizing Nd:BiVO4 nanocrystals (doping amount, 1 at.%), 9.9 mmol of Bi(NO3)3·5H2O and 0.1 mmol of Nd(NO3)3·6H2O were dissolved in 40 mL of deionized water and stirred for 10 min. Then 10 mmol of NH4VO3 dissolved in 8 mL of deionized water was put into the mixture in a three-neck flask. The pH of the admixture was adjusted to 8 with 2 mol L−1 NaOH. The reaction was carried out at 110 °C for 2 h. The products were centrifuged and washed twice utilizing ethyl alcohol, and dried in vacuum at 50 °C. For comparison, pristine BiVO4 nanocrystals were synthesized utilizing the same procedure but without use of neodymium nitrate. EDX spectra, elemental maps and morphologies were characterized
∗
Corresponding authors. E-mail address: [email protected] (Y. Li). 1 These authors contributed equally to this work (co-first authors). https://doi.org/10.1016/j.ceramint.2019.10.014 Received 16 August 2019; Received in revised form 29 September 2019; Accepted 2 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Ruishi Xie, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.014
Ceramics International xxx (xxxx) xxx–xxx
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Fig. 2. TEM images of Nd:BiVO4 nanocrystals.
nanocrystals (Fig. 1b), which are ascribed to the symmetric bending mode δs(VO4), the asymmetric stretching mode and symmetric stretching mode of two different kinds of V–O bonds that present in tetragonal BiVO4 [11,12], respectively. Fig. 2 displays the typical TEM images of Nd:BiVO4 nanocrystals, the subsphaeroidal particles were obtained, and the average size of Nd:BiVO4 nanoparticles is evaluated to be 6 nm. In addition, an interplanar spacing of 0.137 nm is observed, being in conformity with the spacing of the (112) plane of BiVO4. The composition of nanocrystals was studied by EDX spectroscopy (Fig. 3a). The strong peaks from bismuth, vanadium and oxygen, and detectable peaks from neodymium can be seen in the spectrum. In addition, the EDX mapping images demonstrate that bismuth, vanadium, oxygen and neodymium were uniformly distributed in the products (Fig. 3b). For exploring the redox reactions of Nd:BiVO4 nanocrystal battery materials, CV curves for the incipient cycles are acquired at a sweep rate of 0.1 mV s−1. Fig. 4a presents the initial CV curves of Nd:BiVO4 nanocrystal electrode over the potential window of 0.005–3.0 V. For the discharge process, the CV curves display three reduction peaks at around 2.0, 0.95 and 0.5 V, which is due to the intercalation/deintercalation procedure of lithium ions in Nd:BiVO4 nanocrystals. The reduction peak situated at ca. 0.95 V can be attributed to the conversion reaction and intercalation between Li+ and Nd:BiVO4 + − (BiVO4 + xLi + e → LixBiVO4 and LixBiVO4 + (3 − x) Li+ + (3 − x)e− → Bi + Li3VO4). Besides, the broad reduction peak located at ca. 0.5 V can be associated to the reduction of Nd:BiVO4 (BiVO4 → Li3VO4 + Bi → Li3VO4 + LixBi → Li3VO4 + LiBi) and the peak at 2.0 V can be assigned to the generation of the solid electrolyte interface (SEI) stratum on the boundary between the electrolyte and electrode. During the anodic sweep procedure, the oxidation peak situated at 1.1 V is assigned to the phase transition from Bi and Li3VO4 to BiVO4. The weak peak located at ca. 2.7 V can be ascribed to the oxidation of Bi metal into Bi ion. Furthermore, we observed no obvious anodic peaks with respect to the oxidation reactions of Nd to Nd3+ of Nd:BiVO4 nanocrystal, which can be ascribed to its low Nd doping content. Fig. 4b describes the first charge–discharge curves of Nd:BiVO4 nanocrystals at the current density of 0.1 A g−1 at voltages ranging from 0.005 to 3.0 V. We observed three obvious platforms located at 2.0, 0.95 and 0.5 V in the initial discharge cycle procedure, where the platform at 2.0 V is corresponded to the formation of SEI layer, the terrace at 0.95 V is attributed to stepwise lithiation of Nd:BiVO4, while the platform at 0.5 V can be ascribed to the intercalation/de-intercalation procedure of lithium ions in Nd:BiVO4 nanocrystals. As for the first charge cycle, we attribute the obvious platform at 1.1 V to the regained formation of BiVO4. In addition, a less obvious terrace at
Fig. 1. (a) XRD patterns and (b) Raman spectra of Nd:BiVO4 and BiVO4 nanocrystals.
by SEM (Carl Zeiss Ultra 55) and TEM (Carl Zeiss LIBRA 200FE). XRD patterns of the products were obtained by a diffractometer (PANalytical X'Pert PRO) with Cu Kα radiation. Raman spectra were obtained using a Raman apparatus (Renishaw inVia). Electrochemical measurements were carried out using CR2032 coinform cells which were manufactured within an argon-filled glovebox and then aged for 2 days before testing. The synthesized BiVO4-based nanocrystal electrodes acted as the working electrode (anode), Celgard 2500 as the separator, and lithium foil as the counter electrode. The active substance on the electrode was about 1.23 mg cm−2. The electrolyte was 1 mol L−1 LiPF6 in ethylene carbonate–dimethyl carbonate–ethyl methyl carbonate (volume ratio of 1:1:1). The measurements of galvanostatic charge–discharge, cyclic and rate performances were carried out on a NEWARE CT-4000 battery program-control test system at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on an electrochemical workstation (Chenhua CHI720E). 3. Results and discussion XRD patterns of Nd:BiVO4 and BiVO4 nanocrystals (Fig. 1a) show diffraction peaks at 18.3°, 24.4°, 30.7°, 32.7°, 34.8°, 39.5°, and 43.8°, which are indexed to the (101), (220), (211), (112), (220), (301) and (103) planes of the BiVO4 with a tetragonal structure (PDF no. 14–0133). In addition, the peaks of these samples are very similar, indicating that Nd doping does not change the crystal structure of the BiVO4 nanocrystals. The typical vibration bands are observed ca. 365 cm−1, 765 cm−1 and 854 cm−1 for Nd:BiVO4 and BiVO4 2
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Fig. 3. (a) EDX spectrum and (b) elemental mapping images of Nd:BiVO4 nanocrystals.
200 and 100 mA g−1, respectively. A higher capacity for Nd:BiVO4 nanocrystals than that of graphite can still be achieved even with a current rate of 200 mA g−1. What is funny, at a high charge rate of 200 mA g−1, the Nd:BiVO4 nanocrystals exhibit the discharge capacity of ~56% of the initial value. While the current rate returns to 100 mA g−1, the Nd:BiVO4 nanocrystals can quickly recover a capacity of 697 mAh g−1, displaying a high capacity retention. This performance is attractive, revealing the excellent reversibility and rate performance of the Nd:BiVO4 nanocrystals. Memorably, the specific capacity of BiVO4 nanocrystal electrodes is fairly lower than that of Nd:BiVO4 nanocrystal electrode, particularly at high current rates, suggesting that the doping plays a pivotal role in the promotion of rate behavior. The capacities of the BiVO4 nanocrystal are ~15.1, 27.1, 182.4, 296.4 and 600.2 mA h g−1 at the current densities of 2000, 1000, 500, 200 and 100 mA g−1. Markedly, the capacity of Nd:BiVO4 nanocrystal electrode at a current rate of 200 mA g−1 is equivalently ~1.5 times larger than that of BiVO4 nanocrystal electrode. Fig. 4d exhibits the cycling performances of Nd:BiVO4 and BiVO4 nanocrystals at the current rate of 100 mA g−1 for 200 cycles. Capacity retentions of 33.3% and 24.2% after 200 charge/discharge cycles for
2.74 V correlates well with the electrochemical process at the relevant CV peak position. It is interesting that, the Nd:BiVO4 nanocrystals present the specific capacities of 611.4 and 413.2 mAh g−1 for the initial charge and discharge, respectively, with initial coulomb efficiency of 67.6%, which is evidently larger than those of BiVO4 nanocrystals (discharge specific capacity of 424.7 mAh g−1 at 0.1 A g−1 and coulombic efficiency of 66.9%). The findings strongly manifest the noteworthy lithium storage performance of Nd:BiVO4 nanocrystal anode compared with the BiVO4 nanocrystal electrode. In virtue of the electrolyte decomposition and ineluctable emergence of SEI layer, the nanocrystal electrodes experience an initial ineffaceable capacity loss, which are ordinary features for numerous anodic materials. Distinctly, Nd doping can enhance the initial coulombic efficiency and discharge/ charge capacity. This is corresponded to the more effortless diffusion of electrons and lithium ions between the solid particle interface and electrolyte after Nd doping. The rate performance of Nd:BiVO4 and BiVO4 nanocrystal electrodes was assessed at varied current densities. Seen from Fig. 4c, the mean discharge capacities of the Nd:BiVO4 nanocrystals are ~5.7, 22.7, 53.3, 457.8 and 824 mAh g−1 with current rates of 2000, 1000, 500, 3
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Fig. 4. (a) CV curves, (b) Charge/discharge curves, (c) Rate capability, (d) Cycling performance and (e) Nyquist plots of Nd:BiVO4 and BiVO4 nanocrystals.
BiVO4 nanocrystal materials can facilitate its stability in lithium-ion cells. We ascribe this phenomenon to the discrepancy in the ionic semidiameter between Nd3+ and Bi3+. Though their radii are highly semblable, the radius of Nd3+ (0.0995 nm) is a little larger than that of Bi3+ (0.108 nm). When Nd3+ is incoporated into BiVO4 and partially supersedes Bi3+ in the matrix, the lattices of Bi1−xNdxVO4 are relatively small. Hence, the deformation of Bi1−xNdxVO4 is smaller as the Li+ is interposed/deinterposed, and its structure is less easily damaged during charging/discharging. Consequently, the cycling stability of
Nd:BiVO4 and BiVO4 nanocrystals. During the charge/discharge cycling, Nd:BiVO4 nanocrystals possess the highest incipient specific capacity of 627.7 mAh g−1. Nevertheless, the specific capacity presents little degradation after 15 cycles. A specific capacity of merely 182.6 mAh g−1 is kept after 100 cycles, and we notice the capacity maintenance is 29%. BiVO4 nanocrystals can preserve 24% of the incipient specific capacity after 100 cycles, though the original specific capacity is not so large as Nd:BiVO4 nanocrystals. The findings of the cycle performance reveal that the rational incorporation of Nd into the 4
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Provincial Education Department (15ZB0108) and Fund of Southwest University of Science and Technology (18LZX524 and 17LZX543). Dear Yuanli, your Xinyu and Ruishi want to wish you a merry Christmas.
Nd:BiVO4 nanocrystals promotes. To shed more light on the glorious specific capacity and rate performance of Nd:BiVO4 nanocrystals, electrochemical impedance spectroscopy (EIS) measurements were implemented before cycling. The Nyquist plots of Nd:BiVO4 and BiVO4 nanocrystals are shown in Fig. 4e. These samples all display an uncompleted semicircle and a fastigiate line at medium-high and low frequency regions, respectively. The semicircle manifests the double-layer capacitance between the electrolyte–electrode interface and charge-transfer resistance (Rct). In light of the equivalent circuit (the inset of Fig. 4e), Nd:BiVO4 nanocrystal presents a much lower Rct value of 44.1 Ω than that of BiVO4 nanocrystal (48.2 Ω), manifesting the high electrical conductivity of Nd:BiVO4 nanocrystal. The high electrical conductivity of Nd:BiVO4 nanocrystal originates from the Nd:BiVO4 nanocrystal with almost uniform Nd contribution.
References [1] F.Y. Cheng, J. Liang, Z.L. Tao, J. Chen, Functional materials for rechargeable batteries, Adv. Mater. 23 (2011) 1695–1715. [2] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19–29. [3] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928–935. [4] K. Xu, Electrolytes and interphases in Li-ion batteries and beyond, Chem. Rev. 114 (2014) 11503–11618. [5] L.W. Ji, Z. Lin, M. Alcoutlabi, X.W. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682–2699. [6] B. Scrosati, J. Hassoun, Y.K. Sun, Lithium-ion batteries. A look into the future, Energy Environ. Sci. 4 (2011) 3287–3295. [7] N. Nitta, F.X. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today 18 (2015) 252–264. [8] M.V. Reddy, G.V.S. Rao, B.V.R. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (2013) 5364–5457. [9] S. Nikam, S. Joshi, Irreversible phase transition in BiVO4 nanostructures synthesized by a polyol method and enhancement in photo degradation of methylene blue, RSC Adv. 6 (2016) 107463–107474. [10] X.Y. Yang, A.J. Fernandez-Carrion, J.H. Wang, F. Porcher, F. Fayon, M. Allix, X.J. Kuang, Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure, Nat. Commun. 9 (2018) 4484. [11] J.Q. Yu, A. Kudo, Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4, Adv. Funct. Mater. 16 (2006) 2163–2169. [12] R.L. Frost, D.A. Henry, M.L. Weier, W. Martens, Raman spectroscopy of three polymorphs of BiVO4: clinobisvanite, dreyerite and pucherite, with comparisons to (VO4)3-bearing minerals: namibite, pottsite and schumacherite, J. Raman Spectrosc. 37 (2006) 722–732.
4. Conclusions Nd:BiVO4 nanocrystals were fabricated by an effective and simple approach. Nd was doped into BiVO4 host to enhance electronic conductivity and lithium-ion diffusion kinetics, thus promoting the electrochemical performance. When employed as anode materials in lithium-ion batteries, a good cycling stability and high capacity of ~611 mAh g−1 at 100 mA g−1 were delivered. Declaration of competing interest None. Acknowledgments This work was supported by Scientific Research Fund of Sichuan
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Fabrication, structure, electrochemical properties and lithium-ion storage performance of Nd:BiVO4 nanocrystals Ruishi Xie1, Yuanli Li1,∗, Heyan Huang, Li Su, Ling Li, Xiaoqin Pan, Zhiyuan Guo, Fen Luo, Zhicheng Guo∗, Baogang Guo, Fangting Chi∗, Yongjun Ma, Haifeng Liu∗ Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, National Co-Innovation Center for Nuclear Waste Disposal and Environmental Safety, Laboratory for Extreme Conditions Matter Properties, Analytical and Testing Center, Southwest University of Science and Technology, Mianyang, 621010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanostructures Electrochemical properties Lithium ion batteries
Nd:BiVO4 nanocrystals were synthesized by an effective and simple approach. Nd was incorporated into BiVO4 host to enhance electronic conductivity and lithium-ion diffusion kinetics, thus promoting the electrochemical performance. When employed as anode materials in lithium-ion batteries, a good cycling stability and high capacity of ~611 mAh g−1 at 100 mA g−1 were delivered. The work can be extensively employed for fabricating other electrode materials and promoting the electrochemical performance in energy storage correlative domains.
1. Introduction Battery technology is widely recognized as essential for reducing the energy and environmental troubles, since it can be combined with renewable energy and easily utilized in many fields, e.g., renewable energy storage systems, transportation systems and portable electronics [1–3]. In recent years, the booming market for lithium-ion batteries has promptly spread from consumer electronic products to the automotive industry [4–7]. Nevertheless, the finite abundance of lithium-salt brings about the cost increase of lithium-ion batteries and hampers their further development. Furthermore, commercial graphite anode materials possess a low theory capacity, that is not enough to meet the increasing requirement for high power density. Consequently, it is urgent to employ new anode materials for improving the battery technologies in lithium-ion batteries (LIBs) [5,8]. BiVO4 compounds have a framework structure constituted by regular array of angularity-shaped VO4 tetrahedrons and BiO8 dodecahedeons [9,10]. It is a promising anode alternative for LIBs, due to its low poisonousness, high theoretical capacity and abundant elements in nature. Nevertheless, BiVO4 is generally subjected to the aggregation in alloying/dealloying procedure, large volume inflation and low electroconductibility, which brings about discontented rate capability and cycle stability. Great efforts have been made for handling the problems, including designing nano/micro structures, carbon-packing, reducing particle size. These approaches broke down for improving the diffusion coefficient of lithium-ion and intrinsic electronic conductivity. Toward
this end, the combination of nanostructured BiVO4 with incorporating metal ion would be a charming strategy for enhancing the electrochemical performance of BiVO4 LIBs. Here, we demonstrate an effective and simple approach to synthesize Nd doped BiVO4 nanocrystals. Nd was incorporated into BiVO4 host to enhance electronic conductivity and lithium-ion diffusion kinetics, thus promoting the electrochemical performance. A good cycling stability and high capacity of ~611 mAh g−1 at 100 mA g−1 were delivered, which are of tremendous importance for high performance LIBs. The work can be extensively employed for fabricating other metal compounds and promoting the electrochemical performance in energy storage correlative domains. 2. Experimental In a typical experiment, for synthesizing Nd:BiVO4 nanocrystals (doping amount, 1 at.%), 9.9 mmol of Bi(NO3)3·5H2O and 0.1 mmol of Nd(NO3)3·6H2O were dissolved in 40 mL of deionized water and stirred for 10 min. Then 10 mmol of NH4VO3 dissolved in 8 mL of deionized water was put into the mixture in a three-neck flask. The pH of the admixture was adjusted to 8 with 2 mol L−1 NaOH. The reaction was carried out at 110 °C for 2 h. The products were centrifuged and washed twice utilizing ethyl alcohol, and dried in vacuum at 50 °C. For comparison, pristine BiVO4 nanocrystals were synthesized utilizing the same procedure but without use of neodymium nitrate. EDX spectra, elemental maps and morphologies were characterized
∗
Corresponding authors. E-mail address: [email protected] (Y. Li). 1 These authors contributed equally to this work (co-first authors). https://doi.org/10.1016/j.ceramint.2019.10.014 Received 16 August 2019; Received in revised form 29 September 2019; Accepted 2 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Ruishi Xie, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.014
Ceramics International xxx (xxxx) xxx–xxx
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Fig. 2. TEM images of Nd:BiVO4 nanocrystals.
nanocrystals (Fig. 1b), which are ascribed to the symmetric bending mode δs(VO4), the asymmetric stretching mode and symmetric stretching mode of two different kinds of V–O bonds that present in tetragonal BiVO4 [11,12], respectively. Fig. 2 displays the typical TEM images of Nd:BiVO4 nanocrystals, the subsphaeroidal particles were obtained, and the average size of Nd:BiVO4 nanoparticles is evaluated to be 6 nm. In addition, an interplanar spacing of 0.137 nm is observed, being in conformity with the spacing of the (112) plane of BiVO4. The composition of nanocrystals was studied by EDX spectroscopy (Fig. 3a). The strong peaks from bismuth, vanadium and oxygen, and detectable peaks from neodymium can be seen in the spectrum. In addition, the EDX mapping images demonstrate that bismuth, vanadium, oxygen and neodymium were uniformly distributed in the products (Fig. 3b). For exploring the redox reactions of Nd:BiVO4 nanocrystal battery materials, CV curves for the incipient cycles are acquired at a sweep rate of 0.1 mV s−1. Fig. 4a presents the initial CV curves of Nd:BiVO4 nanocrystal electrode over the potential window of 0.005–3.0 V. For the discharge process, the CV curves display three reduction peaks at around 2.0, 0.95 and 0.5 V, which is due to the intercalation/deintercalation procedure of lithium ions in Nd:BiVO4 nanocrystals. The reduction peak situated at ca. 0.95 V can be attributed to the conversion reaction and intercalation between Li+ and Nd:BiVO4 + − (BiVO4 + xLi + e → LixBiVO4 and LixBiVO4 + (3 − x) Li+ + (3 − x)e− → Bi + Li3VO4). Besides, the broad reduction peak located at ca. 0.5 V can be associated to the reduction of Nd:BiVO4 (BiVO4 → Li3VO4 + Bi → Li3VO4 + LixBi → Li3VO4 + LiBi) and the peak at 2.0 V can be assigned to the generation of the solid electrolyte interface (SEI) stratum on the boundary between the electrolyte and electrode. During the anodic sweep procedure, the oxidation peak situated at 1.1 V is assigned to the phase transition from Bi and Li3VO4 to BiVO4. The weak peak located at ca. 2.7 V can be ascribed to the oxidation of Bi metal into Bi ion. Furthermore, we observed no obvious anodic peaks with respect to the oxidation reactions of Nd to Nd3+ of Nd:BiVO4 nanocrystal, which can be ascribed to its low Nd doping content. Fig. 4b describes the first charge–discharge curves of Nd:BiVO4 nanocrystals at the current density of 0.1 A g−1 at voltages ranging from 0.005 to 3.0 V. We observed three obvious platforms located at 2.0, 0.95 and 0.5 V in the initial discharge cycle procedure, where the platform at 2.0 V is corresponded to the formation of SEI layer, the terrace at 0.95 V is attributed to stepwise lithiation of Nd:BiVO4, while the platform at 0.5 V can be ascribed to the intercalation/de-intercalation procedure of lithium ions in Nd:BiVO4 nanocrystals. As for the first charge cycle, we attribute the obvious platform at 1.1 V to the regained formation of BiVO4. In addition, a less obvious terrace at
Fig. 1. (a) XRD patterns and (b) Raman spectra of Nd:BiVO4 and BiVO4 nanocrystals.
by SEM (Carl Zeiss Ultra 55) and TEM (Carl Zeiss LIBRA 200FE). XRD patterns of the products were obtained by a diffractometer (PANalytical X'Pert PRO) with Cu Kα radiation. Raman spectra were obtained using a Raman apparatus (Renishaw inVia). Electrochemical measurements were carried out using CR2032 coinform cells which were manufactured within an argon-filled glovebox and then aged for 2 days before testing. The synthesized BiVO4-based nanocrystal electrodes acted as the working electrode (anode), Celgard 2500 as the separator, and lithium foil as the counter electrode. The active substance on the electrode was about 1.23 mg cm−2. The electrolyte was 1 mol L−1 LiPF6 in ethylene carbonate–dimethyl carbonate–ethyl methyl carbonate (volume ratio of 1:1:1). The measurements of galvanostatic charge–discharge, cyclic and rate performances were carried out on a NEWARE CT-4000 battery program-control test system at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on an electrochemical workstation (Chenhua CHI720E). 3. Results and discussion XRD patterns of Nd:BiVO4 and BiVO4 nanocrystals (Fig. 1a) show diffraction peaks at 18.3°, 24.4°, 30.7°, 32.7°, 34.8°, 39.5°, and 43.8°, which are indexed to the (101), (220), (211), (112), (220), (301) and (103) planes of the BiVO4 with a tetragonal structure (PDF no. 14–0133). In addition, the peaks of these samples are very similar, indicating that Nd doping does not change the crystal structure of the BiVO4 nanocrystals. The typical vibration bands are observed ca. 365 cm−1, 765 cm−1 and 854 cm−1 for Nd:BiVO4 and BiVO4 2
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Fig. 3. (a) EDX spectrum and (b) elemental mapping images of Nd:BiVO4 nanocrystals.
200 and 100 mA g−1, respectively. A higher capacity for Nd:BiVO4 nanocrystals than that of graphite can still be achieved even with a current rate of 200 mA g−1. What is funny, at a high charge rate of 200 mA g−1, the Nd:BiVO4 nanocrystals exhibit the discharge capacity of ~56% of the initial value. While the current rate returns to 100 mA g−1, the Nd:BiVO4 nanocrystals can quickly recover a capacity of 697 mAh g−1, displaying a high capacity retention. This performance is attractive, revealing the excellent reversibility and rate performance of the Nd:BiVO4 nanocrystals. Memorably, the specific capacity of BiVO4 nanocrystal electrodes is fairly lower than that of Nd:BiVO4 nanocrystal electrode, particularly at high current rates, suggesting that the doping plays a pivotal role in the promotion of rate behavior. The capacities of the BiVO4 nanocrystal are ~15.1, 27.1, 182.4, 296.4 and 600.2 mA h g−1 at the current densities of 2000, 1000, 500, 200 and 100 mA g−1. Markedly, the capacity of Nd:BiVO4 nanocrystal electrode at a current rate of 200 mA g−1 is equivalently ~1.5 times larger than that of BiVO4 nanocrystal electrode. Fig. 4d exhibits the cycling performances of Nd:BiVO4 and BiVO4 nanocrystals at the current rate of 100 mA g−1 for 200 cycles. Capacity retentions of 33.3% and 24.2% after 200 charge/discharge cycles for
2.74 V correlates well with the electrochemical process at the relevant CV peak position. It is interesting that, the Nd:BiVO4 nanocrystals present the specific capacities of 611.4 and 413.2 mAh g−1 for the initial charge and discharge, respectively, with initial coulomb efficiency of 67.6%, which is evidently larger than those of BiVO4 nanocrystals (discharge specific capacity of 424.7 mAh g−1 at 0.1 A g−1 and coulombic efficiency of 66.9%). The findings strongly manifest the noteworthy lithium storage performance of Nd:BiVO4 nanocrystal anode compared with the BiVO4 nanocrystal electrode. In virtue of the electrolyte decomposition and ineluctable emergence of SEI layer, the nanocrystal electrodes experience an initial ineffaceable capacity loss, which are ordinary features for numerous anodic materials. Distinctly, Nd doping can enhance the initial coulombic efficiency and discharge/ charge capacity. This is corresponded to the more effortless diffusion of electrons and lithium ions between the solid particle interface and electrolyte after Nd doping. The rate performance of Nd:BiVO4 and BiVO4 nanocrystal electrodes was assessed at varied current densities. Seen from Fig. 4c, the mean discharge capacities of the Nd:BiVO4 nanocrystals are ~5.7, 22.7, 53.3, 457.8 and 824 mAh g−1 with current rates of 2000, 1000, 500, 3
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Fig. 4. (a) CV curves, (b) Charge/discharge curves, (c) Rate capability, (d) Cycling performance and (e) Nyquist plots of Nd:BiVO4 and BiVO4 nanocrystals.
BiVO4 nanocrystal materials can facilitate its stability in lithium-ion cells. We ascribe this phenomenon to the discrepancy in the ionic semidiameter between Nd3+ and Bi3+. Though their radii are highly semblable, the radius of Nd3+ (0.0995 nm) is a little larger than that of Bi3+ (0.108 nm). When Nd3+ is incoporated into BiVO4 and partially supersedes Bi3+ in the matrix, the lattices of Bi1−xNdxVO4 are relatively small. Hence, the deformation of Bi1−xNdxVO4 is smaller as the Li+ is interposed/deinterposed, and its structure is less easily damaged during charging/discharging. Consequently, the cycling stability of
Nd:BiVO4 and BiVO4 nanocrystals. During the charge/discharge cycling, Nd:BiVO4 nanocrystals possess the highest incipient specific capacity of 627.7 mAh g−1. Nevertheless, the specific capacity presents little degradation after 15 cycles. A specific capacity of merely 182.6 mAh g−1 is kept after 100 cycles, and we notice the capacity maintenance is 29%. BiVO4 nanocrystals can preserve 24% of the incipient specific capacity after 100 cycles, though the original specific capacity is not so large as Nd:BiVO4 nanocrystals. The findings of the cycle performance reveal that the rational incorporation of Nd into the 4
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Nd:BiVO4 nanocrystals promotes. To shed more light on the glorious specific capacity and rate performance of Nd:BiVO4 nanocrystals, electrochemical impedance spectroscopy (EIS) measurements were implemented before cycling. The Nyquist plots of Nd:BiVO4 and BiVO4 nanocrystals are shown in Fig. 4e. These samples all display an uncompleted semicircle and a fastigiate line at medium-high and low frequency regions, respectively. The semicircle manifests the double-layer capacitance between the electrolyte–electrode interface and charge-transfer resistance (Rct). In light of the equivalent circuit (the inset of Fig. 4e), Nd:BiVO4 nanocrystal presents a much lower Rct value of 44.1 Ω than that of BiVO4 nanocrystal (48.2 Ω), manifesting the high electrical conductivity of Nd:BiVO4 nanocrystal. The high electrical conductivity of Nd:BiVO4 nanocrystal originates from the Nd:BiVO4 nanocrystal with almost uniform Nd contribution.
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4. Conclusions Nd:BiVO4 nanocrystals were fabricated by an effective and simple approach. Nd was doped into BiVO4 host to enhance electronic conductivity and lithium-ion diffusion kinetics, thus promoting the electrochemical performance. When employed as anode materials in lithium-ion batteries, a good cycling stability and high capacity of ~611 mAh g−1 at 100 mA g−1 were delivered. Declaration of competing interest None. Acknowledgments This work was supported by Scientific Research Fund of Sichuan
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