Wenyuan Liu1, Changfeng Ke1, Xuehai Yan2, Li Duan1*, Lin Li1 and Chong Liu1
1Northwest Institute of Nuclear Technology, Xi’an, Shaanxi 710024, China
2Institute of Process Engineering, CAS, Beijing 100190, China
Received: 13 April, 2015; Accepted: 09 May, 2015; Published: 11 May, 2015
Li Duan, Institute of Process Engineering, CAS, Beijing 100190, China, Email:
Liu W, Ke C, Yan X, Duan L, Li L, Liu C (2015) LiF-MO (M=Co, Fe, Ni) Nanocomposite Thin Film as Anode Materials for Lithium-ion Battery for Lithium-ion Battery. Int J Nanomater Nanotechnol Nanomed 1(1): 014-018. DOI: 10.17352/2455-3492.000004
© 2015 Liu W, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
TiO2 Thin film batteries; Lithium-ion batteries; Nanocomposite materials; Anode; Electrochemical properties
To investigate the electrochemical performance of MO (M=Co, Fe, Ni) nanostructures on lithium insertion and extraction, size-controlled LiF-MO nanocomposite thin-film electrodes, consisting of metallic M and M oxide (MO) nanoparticles in an amorphous, inert LiF matrix, were designed and fabricated using a RF sputtering system with metallic M and LiF mixture targets. The structural and electrochemical properties of nanocomposite thin-film electrodes were characterized using TEM, SAED, XRD, XPS, and electrochemical measurements. The results showed that MO particles with average particle sizes of ca.10nm were well-dispersed in LiF matrix to form a kind of homogeneous LiF-MO nanocomposite by the sputtering method. The inert medium of LiF provides an effective matrix to prevent the crystallization and agglomeration of MO during the deposition and electrochemical cycling of the thin film electrode, and then the well-formed nanophase structure in the nanocomposite thin-film electrodes leads to an excellent electrochemical cycling performance with the stable discharge specific capacity above 300mAh/g.
There has been intense interest in developing new anode materials that store higher densities of lithium for secondary lithium batteries. Rock salt structured MO-type (M = Cu, Fe, Co, Ni) transition metal oxides have been considered as a promising anode because of its high capacity (~700 mAh/g), and excellent recyclibility (up to 100 cycles) toward lithium [11. Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM (2000) Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries. Nature 407: 496-499.-66. Mukherjee R, Krishnan R, Lu T, Koratkar N (2012) Nanostructured electrodes for high-power lithium ion batteries. Nano Energy 1: 518–533.]. Recently, the introduction of nano-sized materials in battery systems has been suggested to be a possibility since the physical, electrical and chemical properties of nano-phases are very different from those of their bulk counterparts [77. Huang XH, Tu JP, Zhang CQ (2010) Hollow microspheres of NiO as anode materials for lithium-ion batteries. Electrochim Acta 55: 8981 - 8985.-99. Nazar LF, Goward G, Leroux F, Duncan M, Huang H, et al. (2001) Nanostructured materials for energy storage. Int J Inorg Mater 3: 191–200.]. It is believed that the key to the successful application of nanostructured electrodes in batteries is a combination of a number of reaction sites, a short transport pathway for electrons and ions, and improving cycle performance of the battery. However, most studies reported previously have focused on bulk materials and few have dealt with the performance of these oxides in thin film electrodes. Thin-film electrodes are ideal models for understanding the relationship between electrode properties and electrochemical behavior because they do not usually contain a binder [1010. Frenning G, Nilsson M, Westlinder J, Niklasson GA, Mattsson M (2001) Dielectric and Li transport properties of electron conducting and non-conducting sputtered amorphous Ta2O5 films. Electrochim Acta 46: 2041–2046.-1212. Zhou YN, Liu WY, Xue MZ, Yu L, Wu CL, et al. (2006) LiF/Co Nanocomposite as a New Li Storage Material. Electrochem Solid-State Lett 9: A147-A150.]. Moreover, thin-film electrodes can be applied directly into thin-film batteries. So far, we have designed and fabricated some kinds of thin-film lithium batteries with excellent electrochemical properties [1313. Liu WY, Fu ZW, Qin QZ (2007) A sequential thin-film deposition equipment for in-situ fabricating all-solid-state thin film lithium batteries. Thin Solid Films 515: 4045-4048.-1515. Huang F, Fu ZW, Chu YQ, Liu WY, Qin QZ (2004) Characterization of Composite 0.5Ag:V2O5 Thin-Film Electrodes for Lithium-Ion Rocking Chair and All-Solid-State Batteries. Electrochem Solid-State Lett 7: A180-A184.]. Currently, MO thin films are typically prepared by reactive sputtering or pulse laser deposition (PLD) using a single metal target [1010. Frenning G, Nilsson M, Westlinder J, Niklasson GA, Mattsson M (2001) Dielectric and Li transport properties of electron conducting and non-conducting sputtered amorphous Ta2O5 films. Electrochim Acta 46: 2041–2046.,1111. Fu ZW, Li CL, Liu WY, Ma J, Wang Y, et al. (2005) Electrochemical reaction of Lithium with Cobalt fluoride thin film electrode. J Electrochem Soc 152: E50-E55.]. The lack of uniformity over a large area is the major drawback for PLD [1111. Fu ZW, Li CL, Liu WY, Ma J, Wang Y, et al. (2005) Electrochemical reaction of Lithium with Cobalt fluoride thin film electrode. J Electrochem Soc 152: E50-E55.]. Therefore, sputtering may be a suitable alternative process for making MO thin films. However, it is difficult to directly fabricated the nanostructured MO thin films with a particles size of less than several tens of nanometers by sputtering due to a plasma heating effect. Furthermore, it may be more important to keep the nanostructured stability to improve the electrode cycling performance during lithium insertion and extraction [11. Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM (2000) Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries. Nature 407: 496-499.-33. Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM (2000) From the vanadates to 3d-metal oxides negative electrodes, Ionics 6: p. 321-330.]. There are some evidences that nanocomposite thin-film electrodes consisting of electrochemical active nano-sized particles embedded into an inactive “buffer matrix” undergo better capacity retention than single thin film during cycling [1616. Kim Il, Blomgren GE, Kumta PN (2003) Nanostructured Si / TiB2 Composite Anodes for Li-Ion Batteries. Electrochem Solid-State Lett 6: A157-A161.,1717. Zhang M, Jia MQ, Jin YH, Shi XR (2012) Synthesis and electrochemical performance of CoO/graphenenanocomposite as anode for lithium ion batteries. Appl Surf Sci 263: 573–578.].
It is well-known that LiF possesses chemical and electrochemical inertia, and may be used as a buffer matrix to compensate for the expansion and shrinkage of metal oxide during electrochemical cycling, thus preventing the aggregation of nanostructured metal oxide during charging/discharging. In this work, we have investigated the preparation and electrochemical properties of LiF-MO, a two-phase nanocomposite thin films electrode, i.e., MO nanodots embedded in a LiF matrix prepared by a sputtering system. It is shown that the addition of inert LiF matrix effectively reduce the particles size of MO and improves the cycling performance of lithium-ion batteries.
Preparation of the nanocomposite thin films
LiF-MO nanocomposite thin films were prepared on the stainless steel substrates by reactive RF magnetron sputtering. A sputtering chamber was evacuated below 5×10-4 Pa with a turbo-molecular pump and a mechanical pump. Composite targets consisting LiF and M ( M= Fe, Co, and Ni) were obtained as targets by cold pressing LiF power (99% Aldrich) and a high pure metal(99.99%) power with the molar ratio of 1:1. Before film deposition, the target was pre-sputtered for 30 minutes to remove the target surface contaminations. The gas mixtures, high purity O2 and Ar with the ratio of 1 to 5, were introduced into sputtering chamber by a mass flow controller. The gas flow was regulated to maintain the chamber pressure at approximately 1.0 Pa. The target-substrate distance was maintained at 60 mm. Sputtering was performed at the RF power of 25 W at room temperature.
Assembly of the Li-ion cells
For the electrochemical measurements, the cells were constructed by using the as-deposited nanocomposites LiF-MO thin films as a working electrode and two lithium sheets as a counter electrode and a reference electrode, respectively. The electrolyte consisted of 1 M LiPF6 in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 (Merck). The cells were assembled in an Ar filled glove box. Charge-discharge measurements were performed at room temperature with a Land BT 1-40 battery test system. The cells were cycled between 0.01 and 3.5 V vs. Li/Li+ at a current density of 28 µA/cm2.
X-ray diffraction (XRD) patterns of the thin film electrodes were recorded by a Rigata/max-C diffractometer with Cu-Kα radiation. The weights of thin films were examined by electrobalance (BP 211D, Sartorius). X-ray photo-electron spectroscopy (XPS) measurements were performed on a Parkin Elmer PHI 6000C ECSA system with monochromatic Al Kα (1486.6 eV) irradiation. To correct possible charging of the films by X-ray irradiation, the binding energy was calibrated using the C1s (284.6 eV) spectrum of hydrocarbon that remained in the XPS analysis chamber as a contaminant. High resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements were carried out by a 200 KV side entry JEOL 2010 TEM.
Results and Discussion
A composition confirmation was carried out by the XPS measurements from as-deposited LiF-MO films. XPS spectra of F 1s peaked at 683.6 eV and Li 1s peaked at 55.6 eV in Figure 1 can be assigned to LiF [1818. Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE (1975) Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp.]. XPS signals from transition metals were also detected. Fe 3p, Co 3p and Ni 3p XPS spectra of as-deposited thin films were shown in Figure 2(a), (b) and (c), respectively. The FeO/LiF spectra contain elemental Fe 2p3/2 and 2p1/2 peaks at 706.6 and 720.4 eV, respectively, close to their previously reported values [1818. Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE (1975) Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp.,1919. Blanchard P, Grosvenor AP, Cavell RG, Arthur M (2008) X-ray photoelectron and absorption spectroscopy of metal-Rich phosphides M2P and M3P (M=Cr−Ni). Chem Mater 20: 7081–7088.]. The high-energy shoulder on the metallic Fe peak at 711 eV and the broad satellite peak centered at about 715 eV are characteristic of FeO. Based on the relative intensities of the Fe and FeO peaks, about 30% of the Fe XPS signal arises from metallic Fe, 70% from FeO. For the CoO/LiF thin film, the XPS spectrum includes Co 2p3/2 and Co 2p1/2 binding energies of 778.5 eV and 794.4 eV, respectively, which can be assigned to metal Co. The high-energy shoulder on the metallic Co peak at 780.2 eV appears to be associated with a single Co oxide, namely CoO. The satellite peaksat 783.5 and 801.9 eV confirm the presence of CoO [1818. Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE (1975) Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp.]. About 35% of the XPS signal was due to metallic Co while 65% was associated with CoO. As was observed in the Fe- based and Co-based films, the metallic Ni 2p3/2 peaks appear close to the expected value of 852.3 eV. The presence of NiO is indicated by the high-energy shoulders on the metallic Ni line at energies of about 854.2 eV and 857.5 eV [1818. Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE (1975) Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp.-2020. Davidson A, Tempere JF, Che M, Roulet H, Dufour G (1996) Spectroscopic studies of Nickel(II) and Nickel(III) species generated upon thermal treatments of Nickel/Ceria-supported materials. J Phys Chem 100: 4919–4929.]. Metallic Ni accounted for about 40% of the XPS signal with about 60% of the signal arising from NiO. From the above results, the MO/LiF nanocomposites are mainly composed of metallic oxide, LiF and a quantity of metal. The existence of metal is helpful to increase the electric conduction of nanocomposites thin films, and then improve the electrochemical properties of thin film electrodes.