A carbon nanotube-reinforced noble tin anode structure for lithium-ion batteries
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A carbon nanotube (CNT)-reinforced noble tin anode structure in which CNTs fasten the tin layer to a copper underlayer has been fabricated using plating techniques so as to improve the cyclability of lithium-ion batteries. In this process, a Cu/CNTs composite layer, on one side of which CNTs protrude from the surface, is formed using a reverse current electrodeposition technique. The surface of this composite layer is subsequently coated with a tin layer by a substitution-type electroless plating technique, resulting in the CNT-reinforced noble tin anode structure. The electrochemical characteristics of this noble tin anode structure have been evaluated and compared to those of a tin anode structure without CNTs. The noble tin anode structure shows significantly improved cyclability compared with the tin anode structure and maintains a higher reversible capacity of 591 mAh g−1, a value that is 1.6 times the theoretical capacity of graphite, even after 30 cycles.
KeywordsLithium-ion battery Tin anode Carbon nanotube Plating Cyclability
Lithium-ion batteries have the highest energy density values among practical battery systems and thus have been widely used as power sources for portable devices and hybrid cars. However, next-generation electric vehicles demand higher performance than is provided by conventional lithium-ion batteries. Presently, graphite is the most common anode material for lithium-ion batteries due to its low cost, availability, and durability, but the practical capacity of graphite has a theoretical limit of 372 mAh g−1. Consequently, alternative anode materials with higher capacities have been researched vigorously. Tin in particular has a theoretical capacity of 994 mAh g−1 and therefore has been investigated as one of the most promising prospective next-generation anode materials . Since tin layers can be formed on copper current collectors by plating, the research and development of electrodeposited tin materials have been the main focus. As opposed to graphite anodes that react with Li+ ions by intercalation, tin anodes react with Li+ ions by alloying. The alloying and dealloying reactions result in a considerable volume change and eventual pulverization of the active tin material, leading to isolation of the tin from the copper current collector during charge–discharge cycling, thus producing poor cyclability. To overcome this problem, the following strategies have all been investigated: the formation of an intermetallic layer between the tin and copper layers by annealing ; the modification of the tin grain size by pulse electrodeposition ; the use of tin electrodeposition to form a multi-layered structure ; Sn-based electrodeposited alloy materials such as Sn–Cu [5, 6, 7], Sn–Ni [8, 9, 10], Sn–Sb [1, 11], and Sn–Ag ; a Sn–Ni alloy three-dimensional structure formed using electrodeposition ; tin and tin-based electrodeposits on three dimensional copper current collectors [13, 14]; an electrodeposited tin film reinforced with copper nanowires ; and electrodeposited tin films on carbon fibers . However, further improvements of both the practical specific capacity and the cyclability of tin-based anodes, as well as the development of practical fabrication processes, are still required.
Since carbon nanotubes (CNTs) [17, 18] have superior mechanical properties, good electro-conductivity, and low density, CNT-reinforced tin anodes are expected to show improvements in anode performance characteristics, especially cyclability. Li et al. have reported that a Sn/CNTs composite film formed by electrodeposition exhibits improved first charge and discharge capacities compared with a tin film, although the capacity decreases with increasing cycle number . Zhao et al. have reported that a Sn/CNTs composite film shows poor cyclability that is almost the same as that of pure tin . In order to enhance the cyclability of tin active material layers, an anode structure in which the adhesion strength between the tin layer and the copper layer are reinforced by fibrous objects such as the CNTs may potentially be effective. We have therefore studied the fabrication of various metal/CNTs composite films, such as Cu/CNTs [21, 22, 23, 24, 25], using plating techniques. In the present study, a CNT-reinforced noble tin anode structure in which the CNTs fasten the tin active material layer and the copper underlayer was produced using a plating technique, and the electrochemical characteristics of the resulting noble anode were evaluated.
2.1 Fabrication of the CNT-reinforced tin anode structure
2.1.1 Formation of the Cu/CNTs composite layer
2.1.2 Formation of the tin layer
The tin layer was fabricated by electroless plating, using Sn2P2O7, K4P2O7, CS(NH2)2, and HCl, all of which were reagent grade. A replacement-type electroless deposition method was employed in this study to form tin layers both on the Cu/CNTs composite layer and on the copper layer. A 0.1 M Sn2P2O7/0.4 M K4P2O7/8 M CS(NH2)2 solution was used as the replacement-type tin electroless deposition bath, the pH of which was adjusted to 5 by the addition of HCl. The electroless deposition of tin was performed at 90 °C over 600 s to produce a tin film with a thickness of about 1 μm. The amount of tin deposited per unit surface area was determined using X-ray fluorescence spectroscopy (XRF, Rigaku ZSX Primus II).
2.2 Characterization of the anode structure
The morphology of each electrodeposited specimen was observed using field emission scanning electron microscopy (FE-SEM, Hitachi Co., Ltd., SU8000). The phase structures of the samples were evaluated by X-ray diffraction (XRD, Shimadzu Seisakusho XRD-6000) with Cu Kα1 radiation.
2.3 Electrochemical measurements
The electrochemical properties of samples were assessed using CR2032-type coin cells. A three-electrode cell configuration, consisting of a fabricated tin electrode (working electrode, WE), Li foil [reference electrode (RE), and counter electrode (CE)], and a 1.0 M solution of LiPF6 in a mixed solvent (ethylene carbonate:diethyl carbonate = 1:1 by volume), was employed. The coin cells were assembled in an argon-filled glove-box. Cyclic voltammetry (CV) was carried out using a computer-controlled electrochemical measurement system (Bio-Logic Co., VSP) and cyclic voltammograms were acquired at a scan rate of 0.05 mV s−1 over the potential range of 0.02–1.5 V (vs. Li/Li+). Charge–discharge measurements were conducted galvanostatically using an automatic charge–discharge instrument (Hokuto Denko Co., HJ1001SD8) at a current density of 100 mA g−1 (a rate of approximately 0.1 C) between 0.02 and 1.2 V (vs. Li/Li+). All electrochemical measurements were performed at room temperature. The specific capacity of each sample was calculated by dividing the quantity of electricity passed through the cell by the mass of the tin.
3 Results and discussion
3.1 Surface morphology of the anode structure
A new tin anode structure for lithium-ion batteries, in which CNTs fasten the copper underlayer and tin active material layer, has been fabricated using plating techniques. CV measurements show that the lithiation mechanism of the new tin anode is different from that of a standard tin anode in the first cycle and that the lithiation rate is improved by the presence of CNTs. This novel tin anode exhibits superior cycling performance and maintains a high reversible capacity of 591 mAh g−1 even after 30 cycles. This superior cyclability is attributed to the fastening effect of the CNTs, in which a portion of each CNT is fixed tightly in the copper underlayer.
This work was supported by a Grant-in Aid for Scientific Research (B) (No. 26289270) from the Japan Society for the Promotion of Science (JSPS).
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