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A facile chemical route to synthesize copper particles-modified LiFe0.95Mo0.05PO4 for lithium-ion batteries
Materials Letters 196 (2017) 4–7
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A facile chemical route to synthesize copper particles-modified LiFe0.95Mo0.05PO4 for lithium-ion batteries Yan Wang a, Zhen-yu He a, Jin-ju Chen a, Kun Liang b, Kyle Marcus b, Zhe-sheng Feng a,⇑ a b
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China Nanoscience Technology Center, University of Central Florida, Orlando, FL 32826, USA
a r t i c l e
i n f o
Article history: Received 2 August 2016 Received in revised form 28 January 2017 Accepted 25 February 2017 Available online 27 February 2017 Keywords: Energy storage and conversion Composite materials Lithium iron phosphate Copper particles Surface modification
a b s t r a c t In this work, a method of fabricating copper particles-modified LiFe0.95Mo0.05PO4 materials by a two-step chemical process is proposed. The structures and morphologies of the samples are characterized by XRD, SEM and XPS. The results demonstrate that Cu0 is the major valence of Cu element, and crystal structure of the electrode material isn’t destroyed after distributing copper particles well on the surface of LiFe0.95Mo0.05PO4. Compared with LiFe0.95Mo0.05PO4, the samples containing Cu exhibit better initial discharge capacity of 143.6 mA h g 1 at 0.1 C and capacity retention of 96.6% after 60 cycles. The results indicate that the enhanced kinetics of electron and lithium-ion transport in the samples is attributed to the superficial copper particles. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Lithium iron phosphate (LiFePO4) has been gaining momentum because of its promising electrochemical properties as cathode materials for lithium-ion batteries, such as high theoretical capacity, stable voltage plateau, cycling stability, environmental compatibility and low cost [1,2]. However, poor electronic conductivity and low lithium-ion diffusivity coefficient limit its applications [3]. Therefore, several approaches have been taken on ion doping [4,5], particle size decreasing [6,7] and surface modification with electronic conductive agents (carbon, metal, etc) [8–16]. It is well known that surface modification is efficient to optimize the electrochemical performance of LiFePO4 [11–15]. However, carbonaceous materials could decrease the tap density and volumetric energy density of LiFePO4 composites, while metal particles (Cu, Ag etc.) will not lessen those properties. Croce et al. firstly reported a physical dispersion approach using copper particles (0.1 mm average size) as coating material, and Cu-added LiFePO4 composites showed a cyclic capacity of 130 mA h g 1 at 0.2 C [13]. Later, Lee et al. incorporated Cu flakes with LiFePO4/C cathode by ball milling compounding [15]. However, the electrochemical improvement of LiFePO4/Cu electrode material is still
⇑ Corresponding author. E-mail address: [email protected] (Z.-s. Feng). http://dx.doi.org/10.1016/j.matlet.2017.02.109 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
limited, since copper particles are difficult to disperse uniformly by conventional physical methods. Herein, we introduced a facile two-step chemical route to synthesize copper particles-modified LiFe0.95Mo0.05PO4. Contrary to those crudely physical mixing procedures, this modified chemical method could disperse copper particles homogeneously. The asprepared material was characterized by a variety of tests and the LiFe0.95Mo0.05PO4/Cu cathode exhibited enhanced performance in both capacities and rate capabilities.
2. Experimental The LiFe0.95Mo0.05PO4 powder was synthesized by solid state reactions of Li2CO3, FeC2O42H2O, NH4H2PO4, and MoO3 in the stoichiometric ratio of 0.5:0.95:1:0.05. After mixing in ethyl alcohol and ball-milling for 10 h, the precursors were heated at 350 °C for 4 h and, then 700 °C for 10 h under nitrogen atmosphere. The LiFe0.95Mo0.05PO4 powder was obtained after cooling down to ambient temperature. The LiFe0.95Mo0.05PO4/Cu composites were prepared by a simple chemical process as described below. A small amount of LiFe0.95Mo0.05PO4 powder was added in Ag+ activating solution, where Ag+ would be reduced and act as catalyze center for Cu ion. After agitating at 40 °C for 3 h, the powder was filtered and desiccated at 100 °C for 3 h. Analytical-grade HCHO (17 ml/L), EDTA-2Na (24 g/L), C4O6H4KNa4H2O (10 g/L), NaOH (11 g/L), and CuSO45H2O (13 g/L) were mixed in deionized water to form chemical plating
Y. Wang et al. / Materials Letters 196 (2017) 4–7
Fig. 1. The XRD patterns of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites. The reflections of LiFePO4 (ICDD PDF No. 40-1499) are shown for comparison.
solution. Following the solution and LiFe0.95Mo0.05PO4 powder were mixed with continuous magnetic stirring at 45 °C for 10 min. Then the sample was washed with deionized water and dried at 100 °C for 2 h in a vacuum oven. After annealing at 400 °C for 3 h under nitrogen atmosphere, the LiFe0.95Mo0.05PO4/ Cu composites were harvested. Details about the characterizations and electrochemical measurements of the synthesized materials can be found in ‘‘Supplementary Information”.
3. Results and discussion The XRD patterns of as-prepared samples are shown in Fig. 1. The dominant diffraction lines of LiFe0.95Mo0.05PO4 and
5
LiFe0.95Mo0.05PO4/Cu composites can be indexed as orthorhombic Pnma space group (ICDD PDF No. 40-1499), indicating a perfect crystalline. Besides, no obvious peaks of impurity phases are found in LiFe0.95Mo0.05PO4 patterns. It proves that low contents of Mo-doping have no effect on the structure of LiFePO4. However, for LiFe0.95Mo0.05PO4/Cu composites, XRD pattern shows a new and weak phase (the peak at 43°) corresponding to Cu. These results indicate that chemical synthesis processes of copper particles do not influence the crystal structure of LiFePO4 and a low amount of copper particles are incorporated into the compound. The SEM images of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites are presented in Fig. 2(a, b). And Fig. 2(c) shows the schematic chemical process of LiFe0.95Mo0.05PO4/Cu composites. Metallic active particles are exposed to form nucleation sites and then small copper particles grow around the nucleation sites on the surface of LiFe0.95Mo0.05PO4. As shown, both composites particles are homogeneous and in micron scale. The irregular LiFe0.95Mo0.05PO4 in Fig. 2(a) consists of sphere-like particles which aggregate slightly with a size distribution of 300–700 nm. The agglomeration of partial particles is due to large surface area and high surface energy. By contrast, the LiFe0.95Mo0.05PO4/Cu composites prepared by chemical processes exhibited in Fig. 2(b) possess well-distributed small size copper particles attached to the surface of LiFe0.95Mo0.05PO4, providing good grain-to-grain electronic contact and reducing the resistance between the particles interfaces even at relatively low concentrations. Moreover, the copper particles are conducive to enhance the electrochemical kinetic property. Note that the LiFe0.95Mo0.05PO4/Cu composites here exhibit a high tap density of 1.46 g cm-3, higher than commercial LiFePO4 material (typically 1.0 g cm 3). The oxidation states of Mo and Cu in LiFe0.95Mo0.05PO4/Cu composites were determined by XPS. As presented in Fig. 3(a), three distinct peaks centered at 228.7 eV, 232.1 eV and 235.5 eV are corresponded with Mom+ (3d5/2), Mo6+ (3d5/2) and Mo6+ (3d3/2), respectively. Mom+ (3d5/2) is an intermediate state between Mo4+ (3d5/2) (229.4 eV) and Mo0 (3d5/2) (227.9 eV). This result reveals that Mo6+ is major and a little intermediate state Mom+ somewhat exists in the LiFe0.95Mo0.05PO4/Cu composites. The valence change
Fig. 2. SEM images of (a) LiFe0.95Mo0.05PO4 and (b) LiFe0.95Mo0.05PO4/Cu composites; (c) schematic of two-step chemical process.
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Y. Wang et al. / Materials Letters 196 (2017) 4–7
Fig. 3. XPS spectra of Mo 3d (a) and Cu 2p (b) for LiFe0.95Mo0.05PO4/Cu composites.
Fig. 4. The first charge-discharge profiles (a) and cycling and rate performance (b) of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites.
of Mo may influence the electronic configuration and generate some electron holes, which are helpful to enhance conductivity. Cu 2p spectra in Fig. 3(b) is split into two parts due to spin-orbit coupling and each part consists of a main peak (932.3 eV) and a shoulder peak (952.5 eV), which are characteristic for Cu0. In addition, two weak satellite peaks at binding energy of approximately 943.1.0 eV and 962.3 eV can be assigned to Cu oxidation. The result indicates that copper particles with two different chemical valences (Cu0 and Cun+) co-exist in the composites. However, the intensities of satellite peaks are much weaker than that of main peaks, indicating that oxidized Cu are much less than Cu0. This result indicates that copper particles in LiFe0.95Mo0.05PO4/Cu composites mainly exist as Cu0. Fig. 4(a) shows the initial charge-discharge profiles of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites at 0.1C. Apparently, the two samples have flat discharge plateaus at 3.4 V compared with the Li/Li+. The initial charge and discharge capacities are 146.2, 133.8 mA h g 1 and 155.8, 143.6 mA h g 1 for the LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu electrodes, respectively, with the corresponding Coulombic efficiencies of 91.5% and 92.2%. Therefore, the LiFe0.95Mo0.05PO4/Cu composites have a better electrochemical performance than LiFe0.95Mo0.05PO4 at low current rate. The cycle performances with different current rates of both composites are exhibited in Fig. 4(b), and both samples show good capacity retention ratio and cyclic stability even at a high current rate of 5 C. In particular, the discharge capacity of LiFe0.95Mo0.05PO4/Cu composites decreases, but remains as high
as 135.9, 126.1, 111.9, and 87.9 mA h g 1 corresponding to 0.5, 1, 2, and 5C, respectively. As for LiFe0.95Mo0.05PO4, the capacity is 127.1, 114.9, 98.9, and 78.1 mA h g 1, respectively. It is worth noting that the nominal capacity of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites retained with 93.6% and 96.6% after 50 cycles with the current density back to 0.5 C, respectively. The good rate performance demonstrates high electronic conductivity and fast lithium ion diffusion in the samples, especially in the LiFe0.95Mo0.05PO4/Cu composites. The higher electrochemical capacity of LiFe0.95Mo0.05PO4/Cu composites can be ascribed to the small copper particles, though a small quantity of them is oxidized. The surface metallic copper particles are beneficial to enhance the electronic conductivity and reduce the resistance around the LiFe0.95Mo0.05PO4/Cu particles, which can decrease the polarization of electrode and improve the electrochemical performance.
4. Conclusions LiFe0.95Mo0.05PO4/Cu materials were successfully synthesized via a two-step chemical process. Results indicated that small size copper particles were well-distributed on the surface of LiFe0.95Mo0.05PO4 and didn’t affect the crystal structure. XPS results verified that Mo6+ and Cu0 were the major valence. LiFe0.95Mo0.05PO4/Cu electrode showed enhanced initial discharge capacity of 143.6 mA h g 1 at 0.1 C and reversible capacity of
Y. Wang et al. / Materials Letters 196 (2017) 4–7
126.1 mA h g 1 at 0.5 C with less than 4% loss of capability after 60 cycles. We believe LiFe0.95Mo0.05PO4/Cu material synthesized by this route is promising in wide applications due to its high capacity, and excellent rate performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.61471106 and No.61271040) and the Fundamental Research Funds for the Central Universities (Grant No.ZYGX2015KYQD059). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.02. 109. References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positive-electrode materials for rechargeable lithium batteries, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [3] P.S. Herle, B. Ellis, N. Coombs, L.F. Nazar, Nano-network electronic conduction in iron and nickel olivine phosphates, Nat. Mater. 3 (2004) 147–152. [4] S. Yoon, C. Liao, X.G. Sun, C.A. Bridges, R.R. Unocic, J. Nanda, S. Dai, M.P. Paranthaman, Conductive surface modification of LiFePO4 with nitrogendoped carbon layers for lithium-ion batteries, J. Mater. Chem. 22 (2012) 4611– 4614.
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[5] Q.L. Liu, W.M. Liu, D.X. Li, Z.X. Wang, Synthesis and characterization of grapelike LiFe0.97M0.03PO4/C (M= Ni, CO, Mn) composites, Mater. Lett. 162 (2016) 87–90. [6] C.W. Sun, S. Rajasekhara, J.B. Goodenough, F. Zhou, Monodisperse porous LiFePO4 microspheres for a high power Li-ion battery cathode, J. Am. Chem. Soc. 133 (2011) 2132–2135. [7] Y.J. Lv, Y.F. Long, J. Su, X.Y. Lv, Y.X. Wen, Synthesis of bowl-like mesoporous LiFePO4/C composites as cathode materials for lithium ion batteries, Electrochim. Acta 119 (2014) 155–163. [8] P.P. Prosini, D. Zane, M. Pasquali, Improved electrochemical performance of a LiFePO4-based composite cathode, Electrochim. Acta 46 (2001) 3517–3523. [9] Z. Zhou, Y. Wang, Q.M. Jonathan Wu, C.-N. Yang, X. Sun, Effective andefficient global context verification for image copy detection, IEEE T. Iin. Foren. Sec. 12 (2017) 48–63. [10] J.K. Kim, J.W. Choi, G. Cheruvally, J.U. Kim, J.H. Ahn, G.B. Cho, K.W. Kim, H.J. Ahn, A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries, Mater. Lett. 61 (2007) 3822–3825. [11] B.F. Zou, Y.Q. Wang, S.M. Zhou, Spray drying-assisted synthesis of LiFePO4/C composite microspheres with high performance for lithium-ion batteries, Mater. Lett. 92 (2013) 300–303. [12] Z.S. Feng, L.L. Wang, Y. Wang, J.J. Chen, Z.Y. He, D.Y. Ji, Y.X. Zhang, Improved electrochemical properties of LiFePO4/Cu cathode material synthesized via an optimized two-step chemical process, J. Alloys Compd. 651 (2015) 712–717. [13] F. Croce, A. D’Epifanio, J. Hassoun, A. Deptula, A. Olczac, A novel concept for the synthesis of an improved LiFePO4 lithium battery cathode, Electrochem. SolidState Lett. 5 (2002) A47–A50. [14] A. Caballero, M. Cruz-Yusta, J. Morales, J. Santos-Peña, E. Rodríguez-Castellón, A new and fast synthesis of nanosized LiFePO4 electrode materials, Eur. J. Inorg. Chem. 9 (2006) 1758–1764. [15] J. Lee, P. Kumar, B.M. Moudgil, R.K. Singh, Electrochemical enhancement of LiFePO4 as a cathode material by incorporating Cu flakes for lithium ion rechargeable battery, Solid State Ionics 231 (2013) 18–24. [16] Z. Zhou, C.-N. Yang, B. Chen, X. Sun, Q. Liu, Q.M. Jonathan, Wu, Effective and efficient image copy detection with resistance to arbitrary rotation, IEICE T. Inf. Syst. E99D (2016) 1531–1540.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A facile chemical route to synthesize copper particles-modified LiFe0.95Mo0.05PO4 for lithium-ion batteries Yan Wang a, Zhen-yu He a, Jin-ju Chen a, Kun Liang b, Kyle Marcus b, Zhe-sheng Feng a,⇑ a b
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China Nanoscience Technology Center, University of Central Florida, Orlando, FL 32826, USA
a r t i c l e
i n f o
Article history: Received 2 August 2016 Received in revised form 28 January 2017 Accepted 25 February 2017 Available online 27 February 2017 Keywords: Energy storage and conversion Composite materials Lithium iron phosphate Copper particles Surface modification
a b s t r a c t In this work, a method of fabricating copper particles-modified LiFe0.95Mo0.05PO4 materials by a two-step chemical process is proposed. The structures and morphologies of the samples are characterized by XRD, SEM and XPS. The results demonstrate that Cu0 is the major valence of Cu element, and crystal structure of the electrode material isn’t destroyed after distributing copper particles well on the surface of LiFe0.95Mo0.05PO4. Compared with LiFe0.95Mo0.05PO4, the samples containing Cu exhibit better initial discharge capacity of 143.6 mA h g 1 at 0.1 C and capacity retention of 96.6% after 60 cycles. The results indicate that the enhanced kinetics of electron and lithium-ion transport in the samples is attributed to the superficial copper particles. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Lithium iron phosphate (LiFePO4) has been gaining momentum because of its promising electrochemical properties as cathode materials for lithium-ion batteries, such as high theoretical capacity, stable voltage plateau, cycling stability, environmental compatibility and low cost [1,2]. However, poor electronic conductivity and low lithium-ion diffusivity coefficient limit its applications [3]. Therefore, several approaches have been taken on ion doping [4,5], particle size decreasing [6,7] and surface modification with electronic conductive agents (carbon, metal, etc) [8–16]. It is well known that surface modification is efficient to optimize the electrochemical performance of LiFePO4 [11–15]. However, carbonaceous materials could decrease the tap density and volumetric energy density of LiFePO4 composites, while metal particles (Cu, Ag etc.) will not lessen those properties. Croce et al. firstly reported a physical dispersion approach using copper particles (0.1 mm average size) as coating material, and Cu-added LiFePO4 composites showed a cyclic capacity of 130 mA h g 1 at 0.2 C [13]. Later, Lee et al. incorporated Cu flakes with LiFePO4/C cathode by ball milling compounding [15]. However, the electrochemical improvement of LiFePO4/Cu electrode material is still
⇑ Corresponding author. E-mail address: [email protected] (Z.-s. Feng). http://dx.doi.org/10.1016/j.matlet.2017.02.109 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
limited, since copper particles are difficult to disperse uniformly by conventional physical methods. Herein, we introduced a facile two-step chemical route to synthesize copper particles-modified LiFe0.95Mo0.05PO4. Contrary to those crudely physical mixing procedures, this modified chemical method could disperse copper particles homogeneously. The asprepared material was characterized by a variety of tests and the LiFe0.95Mo0.05PO4/Cu cathode exhibited enhanced performance in both capacities and rate capabilities.
2. Experimental The LiFe0.95Mo0.05PO4 powder was synthesized by solid state reactions of Li2CO3, FeC2O42H2O, NH4H2PO4, and MoO3 in the stoichiometric ratio of 0.5:0.95:1:0.05. After mixing in ethyl alcohol and ball-milling for 10 h, the precursors were heated at 350 °C for 4 h and, then 700 °C for 10 h under nitrogen atmosphere. The LiFe0.95Mo0.05PO4 powder was obtained after cooling down to ambient temperature. The LiFe0.95Mo0.05PO4/Cu composites were prepared by a simple chemical process as described below. A small amount of LiFe0.95Mo0.05PO4 powder was added in Ag+ activating solution, where Ag+ would be reduced and act as catalyze center for Cu ion. After agitating at 40 °C for 3 h, the powder was filtered and desiccated at 100 °C for 3 h. Analytical-grade HCHO (17 ml/L), EDTA-2Na (24 g/L), C4O6H4KNa4H2O (10 g/L), NaOH (11 g/L), and CuSO45H2O (13 g/L) were mixed in deionized water to form chemical plating
Y. Wang et al. / Materials Letters 196 (2017) 4–7
Fig. 1. The XRD patterns of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites. The reflections of LiFePO4 (ICDD PDF No. 40-1499) are shown for comparison.
solution. Following the solution and LiFe0.95Mo0.05PO4 powder were mixed with continuous magnetic stirring at 45 °C for 10 min. Then the sample was washed with deionized water and dried at 100 °C for 2 h in a vacuum oven. After annealing at 400 °C for 3 h under nitrogen atmosphere, the LiFe0.95Mo0.05PO4/ Cu composites were harvested. Details about the characterizations and electrochemical measurements of the synthesized materials can be found in ‘‘Supplementary Information”.
3. Results and discussion The XRD patterns of as-prepared samples are shown in Fig. 1. The dominant diffraction lines of LiFe0.95Mo0.05PO4 and
5
LiFe0.95Mo0.05PO4/Cu composites can be indexed as orthorhombic Pnma space group (ICDD PDF No. 40-1499), indicating a perfect crystalline. Besides, no obvious peaks of impurity phases are found in LiFe0.95Mo0.05PO4 patterns. It proves that low contents of Mo-doping have no effect on the structure of LiFePO4. However, for LiFe0.95Mo0.05PO4/Cu composites, XRD pattern shows a new and weak phase (the peak at 43°) corresponding to Cu. These results indicate that chemical synthesis processes of copper particles do not influence the crystal structure of LiFePO4 and a low amount of copper particles are incorporated into the compound. The SEM images of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites are presented in Fig. 2(a, b). And Fig. 2(c) shows the schematic chemical process of LiFe0.95Mo0.05PO4/Cu composites. Metallic active particles are exposed to form nucleation sites and then small copper particles grow around the nucleation sites on the surface of LiFe0.95Mo0.05PO4. As shown, both composites particles are homogeneous and in micron scale. The irregular LiFe0.95Mo0.05PO4 in Fig. 2(a) consists of sphere-like particles which aggregate slightly with a size distribution of 300–700 nm. The agglomeration of partial particles is due to large surface area and high surface energy. By contrast, the LiFe0.95Mo0.05PO4/Cu composites prepared by chemical processes exhibited in Fig. 2(b) possess well-distributed small size copper particles attached to the surface of LiFe0.95Mo0.05PO4, providing good grain-to-grain electronic contact and reducing the resistance between the particles interfaces even at relatively low concentrations. Moreover, the copper particles are conducive to enhance the electrochemical kinetic property. Note that the LiFe0.95Mo0.05PO4/Cu composites here exhibit a high tap density of 1.46 g cm-3, higher than commercial LiFePO4 material (typically 1.0 g cm 3). The oxidation states of Mo and Cu in LiFe0.95Mo0.05PO4/Cu composites were determined by XPS. As presented in Fig. 3(a), three distinct peaks centered at 228.7 eV, 232.1 eV and 235.5 eV are corresponded with Mom+ (3d5/2), Mo6+ (3d5/2) and Mo6+ (3d3/2), respectively. Mom+ (3d5/2) is an intermediate state between Mo4+ (3d5/2) (229.4 eV) and Mo0 (3d5/2) (227.9 eV). This result reveals that Mo6+ is major and a little intermediate state Mom+ somewhat exists in the LiFe0.95Mo0.05PO4/Cu composites. The valence change
Fig. 2. SEM images of (a) LiFe0.95Mo0.05PO4 and (b) LiFe0.95Mo0.05PO4/Cu composites; (c) schematic of two-step chemical process.
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Y. Wang et al. / Materials Letters 196 (2017) 4–7
Fig. 3. XPS spectra of Mo 3d (a) and Cu 2p (b) for LiFe0.95Mo0.05PO4/Cu composites.
Fig. 4. The first charge-discharge profiles (a) and cycling and rate performance (b) of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites.
of Mo may influence the electronic configuration and generate some electron holes, which are helpful to enhance conductivity. Cu 2p spectra in Fig. 3(b) is split into two parts due to spin-orbit coupling and each part consists of a main peak (932.3 eV) and a shoulder peak (952.5 eV), which are characteristic for Cu0. In addition, two weak satellite peaks at binding energy of approximately 943.1.0 eV and 962.3 eV can be assigned to Cu oxidation. The result indicates that copper particles with two different chemical valences (Cu0 and Cun+) co-exist in the composites. However, the intensities of satellite peaks are much weaker than that of main peaks, indicating that oxidized Cu are much less than Cu0. This result indicates that copper particles in LiFe0.95Mo0.05PO4/Cu composites mainly exist as Cu0. Fig. 4(a) shows the initial charge-discharge profiles of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites at 0.1C. Apparently, the two samples have flat discharge plateaus at 3.4 V compared with the Li/Li+. The initial charge and discharge capacities are 146.2, 133.8 mA h g 1 and 155.8, 143.6 mA h g 1 for the LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu electrodes, respectively, with the corresponding Coulombic efficiencies of 91.5% and 92.2%. Therefore, the LiFe0.95Mo0.05PO4/Cu composites have a better electrochemical performance than LiFe0.95Mo0.05PO4 at low current rate. The cycle performances with different current rates of both composites are exhibited in Fig. 4(b), and both samples show good capacity retention ratio and cyclic stability even at a high current rate of 5 C. In particular, the discharge capacity of LiFe0.95Mo0.05PO4/Cu composites decreases, but remains as high
as 135.9, 126.1, 111.9, and 87.9 mA h g 1 corresponding to 0.5, 1, 2, and 5C, respectively. As for LiFe0.95Mo0.05PO4, the capacity is 127.1, 114.9, 98.9, and 78.1 mA h g 1, respectively. It is worth noting that the nominal capacity of LiFe0.95Mo0.05PO4 and LiFe0.95Mo0.05PO4/Cu composites retained with 93.6% and 96.6% after 50 cycles with the current density back to 0.5 C, respectively. The good rate performance demonstrates high electronic conductivity and fast lithium ion diffusion in the samples, especially in the LiFe0.95Mo0.05PO4/Cu composites. The higher electrochemical capacity of LiFe0.95Mo0.05PO4/Cu composites can be ascribed to the small copper particles, though a small quantity of them is oxidized. The surface metallic copper particles are beneficial to enhance the electronic conductivity and reduce the resistance around the LiFe0.95Mo0.05PO4/Cu particles, which can decrease the polarization of electrode and improve the electrochemical performance.
4. Conclusions LiFe0.95Mo0.05PO4/Cu materials were successfully synthesized via a two-step chemical process. Results indicated that small size copper particles were well-distributed on the surface of LiFe0.95Mo0.05PO4 and didn’t affect the crystal structure. XPS results verified that Mo6+ and Cu0 were the major valence. LiFe0.95Mo0.05PO4/Cu electrode showed enhanced initial discharge capacity of 143.6 mA h g 1 at 0.1 C and reversible capacity of
Y. Wang et al. / Materials Letters 196 (2017) 4–7
126.1 mA h g 1 at 0.5 C with less than 4% loss of capability after 60 cycles. We believe LiFe0.95Mo0.05PO4/Cu material synthesized by this route is promising in wide applications due to its high capacity, and excellent rate performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.61471106 and No.61271040) and the Fundamental Research Funds for the Central Universities (Grant No.ZYGX2015KYQD059). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.02. 109. References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positive-electrode materials for rechargeable lithium batteries, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [3] P.S. Herle, B. Ellis, N. Coombs, L.F. Nazar, Nano-network electronic conduction in iron and nickel olivine phosphates, Nat. Mater. 3 (2004) 147–152. [4] S. Yoon, C. Liao, X.G. Sun, C.A. Bridges, R.R. Unocic, J. Nanda, S. Dai, M.P. Paranthaman, Conductive surface modification of LiFePO4 with nitrogendoped carbon layers for lithium-ion batteries, J. Mater. Chem. 22 (2012) 4611– 4614.
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