Confinement of nanosized tin(IV) oxide particles on rGO sheets and its application to sodium-ion full cells as a high capacity anode material
Graphical abstract
Introduction
Lithium-ion batteries (LIBs) have been used in a wide variety of applications, including mobile to energy conversion and storage systems, due to their high energy density and excellent cycling stability [1], [2], [3]. This increasing demand of LIBs will potentially lead to a scarcity of lithium resources in the near future. To address this concern, it is necessary to explore cost-effective battery systems as promising alternatives to LIBs; based on the chemistry, availability, and cost, sodium-ion batteries (SIBs) are considered as the most plausible alternative. More importantly, sodium is one of the most abundant resources on earth, attracting considerable attention in recent years, particularly for large-scale energy storage applications.
Graphite is one the most well-known anode materials used in LIBs. However, it can only accommodate for a few sodium ions; this is due to the large ionic radius of Na+ ions and the low intercalation potential of ∼0.1 V vs. Li/Li+, corresponding to −0.2 V vs. Na/Na+ where Na metal plating occurs at the potential. By contrast, hard carbon allows Na+ insertion with the lowest operation voltage and a reasonable capacity of approximately 300 mAh g−1 at very low currents. Apart from the electrode performance, the low operation voltage of hard carbon increases energy density of SIBs. In addition, Na+ insertion-type materials such as TiO2, TiO2(B), Li4Ti5O12, P2-Na0.66Li0.22Ti0.78O2, Na2Ti3O7 and Na2C8H4O4 are based on Ti4+/3+ redox, demonstrating a reasonable cycling stability even though their capacities are lower than that of hard carbon [4], [5], [6], [7], [8], [9], [10]. Large capacities can be obtained through formation of alloy between metal and sodium. Because Si does not readily react with Na above 0 V vs. Na/Na+, investigation towards alloy reactions with Na have mainly focused on Sn, P and Sb [11], [12], [13]. Similar to Li systems, however, formation of alloy usually causes significant volume expansion during the sodiation process. This indicates that degradation phenomena are serious concern in sodium systems.
Tin(IV) oxide is of interest because of its large theoretical capacity of 711 mAh g−1 in Na cells, followed by the conversion reaction: SnO2 + 4Na + 4e− ↔ Sn + 2Na2O [14]. Although there is a large irreversible capacity during the first sodiation process, this can be overcome by the formation of nanosized composites such as nanosized particles, nanotubes, porous nanostructure [15], [16], [17], [18]. However, large surface areas for the electrodes can result in reductive electrolytic decomposition of the interface between the electrode and electrolyte, which thickens the solid electrolytic interphase (SEI) layer [19]. Recent elaboration has been focused to attach active materials onto the inactive matrix, which is used as a mechanical buffer during volume expansion [20]. Those inactive matrixes were suggested, as follows: carbon nanotubes, amorphous carbon, mesoporous carbon, and graphene [21]. Electrode materials can improve their electric conductivities and minimize the byproduct formation by mitigating volume expansion. Furthermore, electron conduction is facilitated by the presence of carbon within the electrode [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33].
Due to the higher theoretical capacity of SnO2 (711 mAh g−1) relative to that of SnO (398 mAh g−1) based on conversion reactions, we synthesized nanosized SnO2 via in-situ decomposition of SnC2O4 during a hydrothermal reaction. In addition, the as-synthesized nanosized SnO2 was attached onto rGO sheets via a self-assembly layer-by-layer process in poly (diallyldimethyl ammonium chloride) solution at 25 °C, utilizing electrostatic adsorption to produce the SnO2/rGO composite. The composite electrode demonstrated remarkable electrode performance, including high capacity, retention, and rate capability. We also studied the reaction process to better understand the suggested conversion reaction in Na cells. Furthermore, we report the conversion of a SnO2/rGO composite anode via pairing with a NaCrO2 cathode in full cells for sodium storage. This report demonstrates that SnO2/rGO composite is suitable for use in SIBs for energy storage applications.
Section snippets
Synthesis of SnO2
SnCl2·2H2O (Kanto), Na2C2O4 (Kanto), and ethylene glycol (Kanto) were dissolved in distilled water and the mixed solution was stirred at room temperature for 6 h. The solution was then poured into a Teflon-lined stainless steel autoclave. The autoclave was hermitically sealed and heated in the temperature range of 130–200 °C for 12 h under autogenous pressure. After the reaction, the autoclave was cooled to room temperature. Finally, the products were washed with deionized water and ethanol and
Synthesis from SnC2O4 to SnO2/rGO composite
The synthetic process of the SnO2/rGO composite is illustrated in Fig. 1a, and the resulting SEM image of the final product is shown in Fig. 1b that the SnO2 particles were confined on the rGO sheets. As shown in Fig. 1c, highly crystalline SnC2O4 was produced via the hydrothermal reaction at 100 °C: SnCl2·2H2O + Na2C2O4 → SnC2O4 + 2NaCl + 2H2O. Here, the resultant was crystallized into the C2/m space group for which all the diffraction peaks are indexed to monoclinic tin oxalate (JCPDS Card
Conclusions
As per schemed, the approach using rGO sheets that allowed confining of SnO2 nanoparticles is significantly effective in improvement on electrochemical properties. Several functionalities of rGO sheets have been demonstrated; electric conductivity raised to ∼10−3 S cm−1 from ∼10−7 S cm−1, fixing the SnO2 nanoparticles, and conduction path that maintains the tin more reversible. These lead satisfactory performances in Na cells, although the conversion and subsequent de-/alloy reactions degrade
Acknowledgements
This research was supported by the National Research Foundation of Korea funded by the Korean government (MEST) (NRF-2015M3D1A1069713 and NRF-2017R1A2A2A05069634).
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