Lithium-ion batteries (LIBs), due to their ability to offer a high capacity and long cyclability have established themselves as a potential high-energy storage device. Numerous developments are constantly being made to overcome the current limitations of LIBs such as volume expansion, diffusion of Li+ ions, etc. Researchers at Graduate School of Convergence Science and Technology and Advanced Institutes of Convergence Technology, Seoul National University, Republic of Korea have synthesized Sn/SnOx-loaded uniform-sized hollow carbon spheres on graphene nanosheets (Sn-UHCS/G) and demonstrated its utility as a lithium-ion battery anode to overcome the limitations in LIBs.
The graphene nanosheets (G) were prepared by thermal reduction of exfoliated graphene oxide in vacuum at 300 °C for 3 h. Uniform-sized carbon coated iron oxides on graphene sheets (C@Iron-oxides/G) were prepared by heating a mixture of Fe(acac)3, oleic acid and graphene nanosheets at 600 °C for 5 h in Ar atmosphere. The C@Iron-oxides/G spheres were etched using 3M HCl for 24 h, rinsed with DI water followed by ethanol and heated at 800 °C for 5 h to obtain uniform-sized hollow carbon spheres/graphene composite (UHCS/G) with a higher conductivity. The UHCS/G was mixed with Sn powder in a weight ratio of 5:5 and heated at 250 °C for 5 h under Ar atmosphere, which enabled loading of Sn in the UHCS/G by melt diffusion to yield Sn-UHCS/G composites. The various steps involved in the synthesis of Sn-UHCS/G is represented in Fig. 1.
Fig. 1 Various steps involved in the synthesis of Sn-UHCS/G
The SEM and TEM images of Sn-UHCS/G indicate uniform-sized hollow carbon spheres (diameter: ~ 10 nm) anchored on graphene nanosheets and the absence of any agglomerated Sn (Fig. 2). The uniform-sized carbon spheres provide a closed structure for Sn that helps to mitigate its direct contact with the electrolyte.
Fig. 2 (a) SEM; and (b, c) TEM images of Sn-UHCS/G
The first and second voltage profile curves of Sn-UHCS/G in the voltage range of 0.01 V to 3.00 V at 0.1 C are shown in Fig. 3(a). Although the specific capacity of Sn-UHCS/G is relatively lower during the 1st cycle due to the formation of SEI on the surface of Sn particles, its capacity is increased during the 2nd cycle. The coulombic efficiency of Sn-UHCS/G is increased from 61.8 % for the initial cycle to 91.2 % during the subsequent cycles. Sn-UHCS/G exhibits a relatively stable capacity retention in the current density range of 0.1 to 3.0 A/g when compared to Sn/G, and pristine Sn (Fig. 3(b)). The discharge capacities of Sn-UHCS/G at 2.0 and 3.0 A/g are ~342 mA h/g and ~275 mA h/g, respectively. Cycling tests performed at 1.0 A/g indicate a relatively stable performance of Sn-UHCS/G for 1000 cycles when compared to Sn/G, and pristine Sn (Fig. 3(c)). The ability of UHCS/G to provide good electronic conductivity through improved coverage of the Sn particles on conductive carbon helps to achieve an improved performance.
Fig. 3 (a) 1st and 2nd discharge/charge curves of Sn-UHCS/G electrode; and (b, c) cycling performance of Sn-UHCS/G, Sn/G, and pristine Sn electrodes.
Sn-UHCS/G exhibited a good rate performance (290 mA/g at 3.0 A/g) and excellent cycle stability (284.1 mA h/g after 1000 cycles at 1.0 A/g). The better electrochemical performance of Sn-UHCS/G is due to the ability of (i) the nanosized Sn/SnOx powders to mitigate the volume expansion during continuous cycles; and (ii) UHCS/G with a high surface area, good electrical conductivity and uniform distribution of Sn/SnOx, which improves diffusion of Li+ ions as well as electrons and promotes diffusion/penetration of electrolyte in the electrode.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Jeongyeon Lee et al., Sn/SnOx-loaded uniform-sized hollow carbon spheres on graphene nanosheets as an anode for lithium-ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.127
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