Synthesis of Co/SnO2 core-shell nanowire arrays and their electrochemical performance as anodes of lithium-ion batteries
- 83 Downloads
The Co/SnO2 core-shell nanowire arrays were synthesized via a simple hydrothermal approach and subsequently the deposition of amorphous SnO2 layer. When used as anode materials of lithium-ion batteries, the Co/SnO2 core-shell nanowire arrays maintain at 667.9 mAh g−1 with the capacity retention of 85.7% after 100 cycles at the current density of 200 mA g−1. For comparison, the discharge capacity of the planar SnO2 electrodes shows the capacity of 196.3 mAh g−1 with the capacity retention of 22.6% after 100 cycles under the same condition. The enhanced electrochemical performance is attributed to the core-shell array nanostructures that can improve the conductivity and buffer the volume changes of tin-based anode during the charge/discharge process.
KeywordsSnO2 Core-shell nanowire array Lithium-ion battery Anode material
Recently, the increasing development of portable electronic equipment and electric vehicles puts forward high requirements for the performance of lithium-ion batteries. As the widely used anode material for commercial lithium-ion batteries, graphite has a relatively low theoretical capacity (~ 372 mAh g−1) [1, 2], which greatly limits the application of lithium-ion batteries in high power density devices. So, a novel anode material which possesses higher specific capacity, better safety performance, longer life, and lower cost is in need. Tin-based materials have been regarded as one of the most promising alternative anodes for lithium-ion batteries [3, 4, 5]. Among them, the low-cost material, SnO2, with the high theoretical reversible capacity (~ 782 mAh g−1) and low electrochemical potential of lithium insertion [6, 7], has been regarded as a promising anode material for the next generation lithium-ion batteries. However, similar to other lithium alloy materials, it suffers ~ 300% volume change during the lithium alloy/dealloy process , which makes it impossible to be popularized on a large-scale production . To circumvent these issues, designing a new structure might be an effective approach. According to previous reports, a wide variety of SnO2 nanostructures, such as nanorods [10, 11], nanoflowers , nanoflakes , porous and hollow structures [14, 15, 16, 17, 18], nanotubes [19, 20, 21] and core-shell structures [22, 23], were synthesized to address the challenge of volume change during cycling processes. Compared with the conventional structure, the array nanostructured electrodes have better volumetric expansion capacity, charge transfer capacity and structural stability [23, 24, 25, 26, 27, 28, 29]. The nano-array structures have several advantages: (1) Nano-sized array anodes can gain better contact between current collector substrate and electrolyte, due to the three-dimensional structure which can offer more spaces for the electrolyte. (2) It can shorten the distance of the diffusion for the ions and electrons, which facilitates the conduction, (3) No binders or conductive additives are needed because of the in situ growth of electrode materials directly onto the current collector substrate. Therefore, the common problems that arise from sluggish charge transfer at the interfaces between the active materials and the binders/additives could be avoided, and this eases the preparation of the electrode structures as well. (4) The better support of array structure contribute to the relaxation of the mechanical stress of electrode materials during the repetitive charge/discharge process [30, 31, 32, 33].
In this report, we demonstrate Co/SnO2 core-shell nanowire arrays with Co nanowires as the current collector and SnO2 as the active material. The good electrical conductivity of Co nanowire matrix can significantly improve the charge transfer capacity of the electrode [34, 35]. Moreover, SnO2 deposited on the Co nanowires has a high specific surface area, which not only increases the contact area of electrode/electrolyte, but also reserves good strain accommodation for anode materials during charge-discharge cycles. Therefore, the three-dimensional core-shell Co/SnO2 nanowire arrays are believed to show good electrochemical performance as anode materials of lithium-ion batteries.
Synthesis of Co nanowire arrays on Ti substrates
The Co nanowire arrays were grown on Ti substrates by the hydrothermal method described by a previous paper with slight modification . The steps were as follows: 5 mmol Co(NO3)3·6H2O and 25 mmol urea were dissolved in a 50-mL deionized water followed by stirring until completely dissolved, and the homogeneous solution and Ti sheets were then transferred into a 100-mL customized hydrothermal reactor, reacting at 95 °C for 3.5 h. After the reaction, the product was rinsed with deionized water and ethanol for several times. Afterwards, it was placed in the tube furnace and kept at 400 °C for 2 h in an argon atmosphere. Finally, the sample was heated at 350 °C for 3 h in hydrogen and/or argon atmosphere and then cooled down to room temperature, and Co nanowire arrays grown on Ti substrates were obtained.
Synthesis of Co/SnO2 core-shell nanowire arrays
Core-shell Co/SnO2 nanowire arrays were deposited onto the substrate by RF sputtering with a 99.9% SnO2 target. The working pressure was 1.0 Pa, and the power was 72 W.
Material characterizations and electrochemical measurement
The morphology and structure of the product were characterized by using X-ray diffraction (XRD, X'PERT, PRO), field emission scanning electron microscope (SEM, S4800, Hitachi), and transmission electron microscope (TEM, Tecnai, G2F30).
CR2025 coin cells, with a metal lithium as the counter electrode and 1 M solution of LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) as an electrolyte, determined the electrochemical properties of the products. Nickel foam was the supported material, while Celgard 2300 porous composite membrane was a diaphragm of the cell. The assembling of CR2025 coin cells was in a glove box in a high purity argon atmosphere with the lower than 0.1 ppm content of oxygen and water.
Galvanostatic cycling test of the assembled cells was carried out on a Land CT2001A system (Wuhan Blue Electric Technology Co. Ltd) in the potential range of 0.01~3.0 V at a current density of 200 mA/g with the temperature of 25 ± 1 °C.
Results and discussion
Therefore, the peaks of SnO2 do not exist after the first cycle, which are replaced by the oxidation/reduction peaks of Sn. The transformation of SnO2 to Sn and the irreversibility of Li2O are the main reason for the low coulombic efficiency in the first cycle . After the first cycle, the curves of each cycle are smooth and almost coincide, which indicate the good reversibility lithiation/delithiation in the subsequent cycles.
Figure 6b shows galvanostatic charge/discharge curves of the as-synthesized materials with the sputtering time of 15 min for the 1st, 2nd, and 100th cycles with the potential window between 0.01~3.0 V, at the current density of 200 mA g−1. There is a “platform” near 0.9 V, which corresponds to the formation of SEI and the irreversible reaction of Li with the SnO2. This is consistent with the characteristics of the CV curve shown in Fig. 6a. Meanwhile, the different discharge capacity of the first two cycles is related to the irreversible Li2O. After 100 cycles, the discharge capacity is still ~ 700 mAh g−1, indicating the good cycling stability.
Figure 8b presents the EIS (electrochemical impedance spectroscopy) images of Co/SnO2 core-shell arrays and planar SnO2 films, with the amplitude of 1 mV and the frequency range from 600 to 0.01 Hz. Both of the two curves show the similar shape of Nyquist plots, composed of a depressed semicircle where a high-frequency semicircle and an inclined line in the low-frequency region . The illustration insertion shows the equivalent circuit of the EIS impedance simulation. Rs represents the internal impedance of the tested LIBs, while Rct and CPE1ct corresponded to charge-transfer resistance and constant phase element of an interface between electrode and electrolyte, respectively. Wo is associated with the Warburg impedance corresponding to the Li-ion diffusion process. As shown in Fig. 8b, the semicircle on the medium-frequency region corresponds the charge-transfer resistance Rct and CPE1ct of the electrode/electrolyte interface, and the inclined line in the low-frequency region corresponds to the lithium-ion diffusion process within the electrode materials. The fitted Rct quantitative values of Co/SnO2 nanowire arrays and planar SnO2 films are 270 and 175 Ω, respectively, indicating that the Co nanowire arrays can be acted as current collector network to improve the electrical conductivity and rapid the electron transport during lithiation/delithiation process, resulting in significant improvement in electrochemical performance of Co/SnO2 nanowire arrays anode. The introduction of metal Co nanowire arrays decreases the distance between electron and collector; thus, the contact impedance and charge transfer impedance are reduced effectively.
Co/SnO2 nanowire arrays were synthesized via a simple hydrothermal approach, thermal heat treatment, and RF sputtering approaches. When used as anode materials of lithium-ion batteries, the Co/SnO2 nanowire arrays maintain at 667.9 mAh g−1 with the capacity retention of 85.7% after 100 cycles at the current density of 200 mA/g, which was much better than planar SnO2 film anode. The enhanced electrochemical performance is attributed to the core-shell array nanostructures that can improve the conductivity and buffer the volume changes of tin-based anode. The effect of sputtering time on the morphology and properties of SnO2 layer was also studied, and the sputtering time of 15 min performed the best cyclic performance.
The authors would like to thank the financial supports from the Natural Science Foundation of China (Grant Nos. 5 61721005).
- 3.Chen LB, Yin XM, Mei L, Li CC, Lei DN, Zhang M, Li QH, Xu Z, Xu CM, and Wang TH (2012) Mesoporous SnO2@carbon core-shell nanostructures with superior electrochemical performance for lithium ion batteries [J]. Nanotechnology 23(3):Google Scholar
- 42.Zhu CR, Xia XH, Liu JL, Fan ZX, Chao DL, Zhang H, Fan HJ (2014) TiO2 nanotube @ SnO2 nanoflake core-branch arrays for lithium-ion battery anode [J]. Nano Energy 41:05–112Google Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.