ARTICLE pubs.acs.org/JPCC
Enhanced Electrochemical Performance of Sn Co Nanoarchitectured Electrode for Lithium Ion Batteries Zhijia Du and Shichao Zhang* School of Materials Science and Engineering, Beihang University, Beijing, China
bS Supporting Information ABSTRACT: Nanoarchitectured Sn Co alloy electrode was prepared via a facile two-step electrodeposition. With uniform Ni nanocone-array as the substrate, Sn Co alloy was deposited for 5 min, and densely packed cylinders were formed with semiglobular top. In this configuration, these Ni cones functioned as structure support, electron transport paths, and the inactive confining buffer. Meanwhile, the space between adjacent Sn Co cylinders as well as the inactive Co matrix accommodated the volume change and cushioned the concomitant internal stress. The nanoarchitectured Sn Co electrode showed a high discharge capacity of ∼650 mAh g 1, which maintained well with capacity retention of 97.3% after 70 cycles and 83.4% after 90 cycles. It also exhibited attractively high rate capability, delivering high-level capacities at various rates with little capacity decay. These remarkable performances of nanoarchitectured Sn Co electrode indicated the potential of its application as anode materials for high-performance lithium ion battery.
’ INTRODUCTION The development of high-performance Li-ion battery is of great technological importance for applications such as portable electronics and electric/hybrid vehicles in the last two decades.1,2 Extensive research efforts have been devoted to substitute tin for graphite as the anode material because of its high theoretical capacity of 994 mAh g 1.3,4 Initially, many attempts had been made to achieve the implementation of pure tin metal,5,6 but they turned out to be unsuccessful because of poor cyclability. Enormous volume change (>300%) occurred in tin electrode during lithiation/delithiation process,7 which led to structural pulverization and electrical disconnection that resulted in severe capacity fading. To overcome these drawbacks, one promising strategy is to design Sn-based intermetallics or composites to buffer the huge volume change and accomplish improved cycle performance. The main idea of preferring intermetallics or composites to pure tin is this: in the lithiation process, tin reacts with lithium to storage Li in LixSn form, whereas the other metal or component serves as the inactive buffer matrix to alleviate the volume expansion of LixSn.3 Various Sn-based intermetallics have been proposed and used as anode materials such as Sn Cu,8 Sn Fe C,9 Ni Sn,10 Sn C Ni,11 and so on. Sn Co alloys have also received several research efforts by various methods, preferably electrodepositon.12 15 These investigations showed limited improvement of the capacity retention, but the capacities all faded gradually within 50 cycles. Consequently, improvement of cycle performance in prolonged duration is urgently required in future research. r 2011 American Chemical Society
It is now realized that another effective approach to improve the cyclability and rate capability of Sn-based electrodes is layout of specific nanostructured electrode, as discussed in previous report.16,17 The nanoarchitectured electrode configurations enlarged the effective surface area of the electrode; meanwhile, they are very suitable to relieve the mechanical and structural strain during charge/discharge process. In a recent communication,18 we have also developed a unique Si architecture supported by Ni nanocone arrays composed of many nanoscale cylinders as the anode material for Li-ion battery. In this configuration, the Ni nanocone arrays facilitated charge collection and transport, supported the electrode structure, and functioned as inactive confining cushions. These nanostructured Si electrodes showed excellent cycle stability and rate capability. Compared with silicon, tin possesses high conductivity and low-cost preparation technology, which can avoid high voltage, high temperature, and high vacuum in Si preparation procedure as reported in our communication.18 Here we report the detailed preparation and characterization of nanoarchitectured Sn Co electrode. The serried Ni nanocones and the subsequent Sn Co alloy coating are both accomplished by electrodeposition, which was thought to not only be a simple and convenient method for commercial process but also ambient and low-powered operation to facilitate the construction of resource saving society. It is shown that the Received: June 25, 2011 Revised: October 19, 2011 Published: October 21, 2011 23603
dx.doi.org/10.1021/jp205979m | J. Phys. Chem. C 2011, 115, 23603–23609
The Journal of Physical Chemistry C
ARTICLE
nanostructured Sn Co electrode exhibits appealing advancement of Li+ storage property, delivering high capacity, long cycle lifespan, and high rate capability.
’ EXPERIMENTAL SECTION Ni nanocone-arrays were fabricated as described in our previous report.18 The deposition current density was fixed at 5, 7.5, 10, and 15 mA cm 2 to investigate the morphology variation of Ni nanocone-arrays. Afterward, the electrodes were prepared by electrodepositing Sn Co alloy onto Ni nanoconearrays. The electrolytic bath consisted of analytical pure Na2SnO3 3 3H2O 0.3 M, CoCl2 3 6H2O 0.02 M, potassium sodium tartrate 0.5 M, and potassium pyrophosphate 0.06 M. Pt foil was used as anode, and the electrolytic solution was continually stirred by a magnetic bar. The electrodeposition of Sn Co alloy was carried out with a current density of 5 mA cm 2 and a bath temperature of 45 °C. For comparison, electrodeposition of Sn Co alloy onto flat Ni-layer-coated Cu foil was also conducted under the same conditions. The above-mentioned flat Ni layer was electrodeposited onto Cu foil from the bath consisted of 1 M NiCl2 3 6H2O and 0.5 M H3BO3 under 25 mA cm 2 for 5 min. After deposition, the obtained electrodes were washed with distilled water and acetone successively and then dried at 60 °C under vacuum environment. The mass of the Sn Co coating was calculated by measuring the substrates before and after the Sn Co electrodeposition via a Mettler AB135-S analytical balance. The morphologies, composition, and phase structure of the electrodes were investigated by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800 SEM system) and MAC Science X-ray diffraction (XRD, Cu Kα radiation). Electrochemical charge discharge behaviors were investigated in half-cells assembled with the as-prepared anode, lithium foil, and Celgard 2300 membrane in an Ar-filled glovebox (MB10-G with TP170b/mono, MBRAUN). LiPF6/EC DEC (1 M, 1:1 by vol.%) was used as the electrolyte. Each cell was aged for 20 h at room temperature before commencing the electrochemical tests. The galvanostatic charge discharge measurements were carried out in a battery test system (NEWARE BTS-610, Newware Technology, China) at room temperature with cutoff voltage of 0.05 to 1.5 V. Cyclic voltammogram (CV) test was performed between 0 and 3.0 V (vs Li/Li+) at 0.1 mV s 1. After the electrodes were charged to 1.5 V (vs Li/Li+), then left on open-circuit for 2 h to obtain equilibrium, electrochemical impedance spectroscopy (EIS) measurements were carried out by applying a sine wave of 5 mV amplitude over a frequency range of 100.00 kHz to 0.01 Hz. CV and EIS were both performed via a Zahner electrochemical workstation (iM6ex). ’ RESULTS AND DISCUSSION Figure 1 depicts a representative preparation procedure of the nanoarchitectured Sn Co electrode. The robust Ni nanocones are first fabricated as the substrate to collect the charge and support the electrode structure. Then, Sn Co alloy is deposited on these Ni nanocones to serve as active material to react reversibly with Li+. The appearance of the electrode including the thickness of the Sn Co alloy can be facilely adjusted by setting different duration time of the electrodeposition,as illustrated in Figure 1. The matrix of Co in Sn Co alloy is thought to work as the inactive confining buffer to sustain the stress derived from the volume expansion in cycling.
Figure 1. Schematic diagram illustrating the fabrication of nanoarchitectured Sn Co electrode: (a) nickel nanocone-arrays before Sn Co electrodepostion, (b) after Sn Co electrodeposition for a short time, and (c) after Sn Co electrodeposition for an extended time.
Figure 2. XRD patterns of (a) Ni nanocone-array and (b) after Sn Co electrodepostion for 10 min.
Figure 2 shows the XRD patterns of the Ni nanocone-array and the obtained electrode after Sn Co alloy electrodeposited for 5 min. In Figure 2a, the peaks at 44.7 and 52.2° are assigned to (111) and (200) of face-centered cubic nickel (JCPDS no. 040850), whereas the peaks at 43.4 and 50.5° are in accordance with (111) and (200) of face-centered cubic copper (JCPDS no. 040836). After Sn Co electrodeposition, the pattern is almost the same as Figure 2a except the evolution of one peak at 30.3°. However, the peak was not simply attributed to some Sn Co alloy components such as Co3Sn2 (JCPDS no. 26-0490), CoSn (JCPDS no. 65-6225), CoSn2 (JCPDS no. 65-5843), and αCoSn3 (JCPDS no. 48-1813). Previous reports showed that metastable phase could be probably formed in Sn Co alloy.15,19,20 Moreover, energy-dispersive X-ray spectroscopy (EDX) was carried out to investigate the elemental composition of the deposit, which verified the existence of Sn and Co with a ratio of 66.4:33.6 (by atomic %). Therefore, it was considered that the Sn66.4Co33.6 would be referred to as a metastable phase. Abundant research confirmed that the most stable electrochemical performance was found in Sn1-xCox alloy for 0.28 < x
