Article pubs.acs.org/JPCC
Poly(3,4-ethylenedioxythiophene) Sheath Over a SnO2 Hollow Spheres/Graphene Oxide Hybrid for a Durable Anode in Li-Ion Batteries Akkisetty Bhaskar,† Melepurath Deepa,*,‡ M. Ramakrishna,§ and T. N. Rao∥ †
Department of Materials Science and Engineering and ‡Department of Chemistry, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram, Andhra Pradesh 502205, India § Centre for Mechanical and Microstructural Characterization and ∥Centre for Nanomaterials, ARCI, Hyderabad, Andhra Pradesh 500005, India S Supporting Information *
ABSTRACT: SnO2 hollow spheres (HSs) were synthesized by a hydrothermal route by use of an organic additive (2mercaptopropionic acid or MPA) and a cationic surfactant (cetyltrimethyl ammonium bromide or CTAB). The progressive transformation of SnO2 solid spheres to SnO2 HSs 140−150 nm in dimensions wherein a thin shell of densely packed SnO2 crystallites with a tetragonal crystal structure surrounds an empty core was followed by scanning- and transmission-electron microscopy. The roles of MPA as the HS structure-directing agent, CTAB as the moiety which prevents HS aggregation, and water as the solvent crucial for hollow core formation were independently determined by elaborate morphological analyses. With the goal of realizing superior electrochemical performance, hybrids of optimized SnO2 HSs embedded in graphene oxide (GO) nanosheets and enveloped by a sheath of a conducting polymer, poly(3,4-ethylenedioxythiophene) or PEDOT, were also synthesized; the continuity of the amorphous PEDOT coating on SnO2 HS/GO was confirmed by elemental mapping and X-ray photoelectron spectroscopy. Galvanostatic charge−discharge studies revealed an initial reversible capacity of 990 mA h g−1 for SnO2 HSs at a current density of 100 mA g−1, and a capacity of 400 mA h g−1 was retained after 30 cycles. A significant improvement in cycling performance was achieved in the SnO2 HS/GO/PEDOT hybrid, as the synergy between the moderately high intrinsic electronic conductivity of GO nanosheets and the ability of PEDOT to buffer the volume change during repetitive Li+ charge−discharge more efficiently compared to pristine SnO2 HS impart a capacity of 608 mA h g−1 at a current density of 100 mA g−1 to the hybrid, retained at the end of 150 cycles, and the latter value was ∼1248 mA h g−1 when the mass of only the SnO2 HS in the hybrid was considered. The SnO2 HS/GO/PEDOT hybrid also showed an excellent rate capability as a capacity of 381 mA h g−1 was attained even at a high current density of 2000 mA g−1. We demonstrate the viability of the SnO2 HS/GO/ PEDOT hybrid as a durable high performance anode for Li-ion batteries. significant ∼300% increase in volume, and upon repeated cycling, the SnO2 active electrode can undergo pulverization which causes breakdown in electrical contact pathways between adjacent particles, leading to rapid capacity decline.4−9 To counter the issue of fast capacity fading in SnO2, researchers in the past have attempted SnO2 nanostructures as electrodes; in SnO2 hollow spheres, cores/shells, nanowires, nanotubes, nanosheets, and mesopores as in nanosized SnO2 the volume change caused by the mechanical stress induced by lithium
1. INTRODUCTION The development of novel anode materials capable of delivering high energy density and cycling stability as alternates to traditional graphite electrodes continues to be a formidable challenge in lithium-ion battery (LiB) research.1−3 Tin oxide (SnO2) is considered to be an attractive alternative to graphite because it is cheap, offers exceptionally large theoretical specific capacity (782 mA h g−1), and has a low Li-ion intercalation potential. Further, SnO2 when employed as an anode in a Liion battery can undergo an irreversible conversion to Sn followed by a reversible alloying with lithium metal to yield a Li4.4Sn alloy, wherein each Sn can react with 4.4 Li atoms. During this reversible alloying reaction Sn undergoes a © 2014 American Chemical Society
Received: December 9, 2013 Revised: March 12, 2014 Published: March 17, 2014 7296
dx.doi.org/10.1021/jp412038y | J. Phys. Chem. C 2014, 118, 7296−7306
The Journal of Physical Chemistry C
Article
intercalation is tolerated more easily than it is in bulk SnO2.5−23 Another alternate approach to increase cyclability involves the preparation of hybrid electrodes of SnO2 with carbon coating or with reduced graphene oxide or carbon nanotubes or conducting polymers. The additive (be it a carbon nanostructure or a conducting polymer) buffers the volume change incurred by the SnO2 active phase, prevents the overall agglomeration of SnO2 particles, and also improves the electrical conductivity properties of the anode and thus reduces the capacity fading of the electrode. The second strategy has been reasonably successful in yielding anodes capable of sustained electrochemical cycling response.24−38 In the past there have been several reports on SnO2− graphene composites.27,28,32−38 Previously, a composite of SnO2/graphene nanosheets prepared by chemical synthesis showed an initial reversible capacity of 541.3 mA h g−1, and after 35 cycles a capacity of 377.3 mA h g−1 was retained at a current density of 200 mA g−1 in the voltage range of 0.01−1.5 V.32 In another study a SnO2/graphene composite consisting of 60 wt % SnO2 nanocrystals gave an initial reversible capacity of 786 mA h g−1 (at 0.5 C rate) in the voltage range of 0.02−3 V. After 50 cycles, the reversible specific capacity reduced to 558 mA h g−1, showing 71% retention of its initial capacity.33 Park et al. reported a SnO2/graphene composite containing 70 wt % of SnO2 prepared by a hydrothermal method which exhibited a charge capacity of 819 mA h g−1 for the first cycle and retained a capacity of 626 mA h g−1 after 50 cycles at a current density of 100 mA g−1.34 In another report on a SnO2/poly(aniline)/ reduced graphene oxide nanocomposite prepared hydrothermally, an initial specific capacity of 784 mA h g−1 faded to 573.6 mA h g−1 with 73% retention, at the end of 50 cycles. The above observed capacity was measured at a 0.2 C rate in the voltage range of 0.01−3 V.35 Recently, Lin et al. prepared a composite of graphene nanoribbons and SnO2 nanocrystals by a chemical method which gave an initial reversible capacity of 1130 mA h g−1, and after 50 cycles a capacity of 825 mA h g−1 was retained (at 100 mA g−1 current density in the voltage range of 0.01−2.5 V).36 Graphene-based mesoporous SnO2 prepared by Yang and co-workers by a hydrothermal method showed an initial discharge capacity of 1595.6 mA h g−1, and after 50 cycles a capacity of 847.5 mA h g−1 was retained. This electrochemical response was measured in the voltage range of 0.01−3 V at a 0.1 C rate.37 Despite significant advances in SnO2/graphene hybrids as elaborated above, hollow structures of SnO2 are advantageous because the local empty space in the structures can partially accommodate the large volume change, delaying capacity fading and imparting efficient catalytic activity and structural stability.9−16 Although manipulation of SnO2 nanostructures either by shape or size control or by inclusion of a conductive inert moiety for better battery performance had shown some promising results in the past, but much room remains for systematically engineering high-capacity SnO2-based electrodes endowed with cycling stability by use of low cost precursors and a cost-effective methodology. Here we present a hydrothermal synthesis of SnO2 hollow spheres using an organic bifunctional molecule followed by formation of a hybrid electrode with graphene oxide and PEDOT. While graphene oxide (GO) nanosheets serve as a scaffold for entrapment of SnO2 hollow spheres and prevent their coalescence during electrochemical cycling, the PEDOT overlayer being a robust conducting polymer effectively inhibits the disintegration of SnO2 HS during charge/discharge cycling, and cumulatively
both GO and PEDOT afford capacity retention with cycling. The conducting polymer thus offers a good tradeoff between mechanical stability and electrical conductivity to the active electrode. We observed that the performance of the binary SnO2 HS/GO and SnO2 HS/PEDOT electrodes was rather dismal relative to the SnO2 HS/GO/PEDOT electrode indicating that both the carbon nanostructure and PEDOT cooperatively control the electrochemical activity of the electrode. We also studied the effect of the organic molecule proportion on the nanostructure and electrochemical response of pristine SnO2 HSs and using process parameters optimized for the high performance SnO2 electrode, hybrid electrodes of SnO2 HS/GO/PEDOT were fabricated by in situ polymerization of the monomer in a SnO2 HS/GO formulation. We provide mechanistic insights into the formation of high performance SnO2 and also present a structure−property correlation by linking cyclability, rate capability, and capacity with structural and interfacial charge transfer and transport aspects. To the best of our knowledge this is the very first report on a SnO2 HS/GO/PEDOT composite wherein the highly conductive matrix of GO coupled with an electrochemically stable conducting polymer efficiently enhances the capacity retention capability of SnO2 and easily outperforms pristine SnO2 anodes.
2. EXPERIMENTAL SECTION 2.1. SnO2 Hollow Spheres (HSs) and Nanostructures. Hollow spheres of SnO2 were prepared by a one-step hydrothermal synthesis route using water as solvent. Typically, in a 100 mL beaker, 1.1 g of 3-mercaptopropionic acid (MPA, Alfa Aesar, 98%) was dissolved in 30 mL of ultrapure water (Millipore) and stirred for 5 min. To this clear solution, 3.324 mmol of SnCl2·2H2O (Merck, 98%) was added and stirred for 10 min, and a solution with SnCl2·2H2O:MPA in a 1:3 molar ratio was obtained. To the resulting colorless clear solution was added 100 mg of cetyltrimethyl ammonium bromide (CTAB, Alfa Aesar, 98%) surfactant followed by 20 mL of Millipore water, and the solution was stirred at 1000 rpm for 2 h. The resulting solution was transferred to an 80 mL Teflon-lined stainless steel autoclave. After heating at 160 °C for 12 h in an oven, the resulting precipitate was filtered and washed with water and dried at 80 °C for 6 h. The obtained ash colored powder was heated in an electric furnace at 500 °C for 5 h which resulted in pure SnO2 hollow spheres or HSs. Similarly solutions of (a) SnCl2·2H2O and MPA mixed in a 1:1 molar ratio with 100 mg of CTAB and (b) only using SnCl2·2H2O and MPA (1:3 molar ratio) without using CTAB and (c) only using CTAB and without MPA from an aqueous solution (50 mL) containing 100 mg of CTAB surfactant and 3.324 mmol of SnCl2·2H2O were also processed separately to produce SnO2, while maintaining all other parameters constant. 2.2. SnO2 HS/GO Hybrid. Graphene oxide (GO) was synthesized from graphite (Aldrich, particle size
