Rational design of Sn-based multicomponent anodes for high performance lithium-ion batteries: SnO₂@TiO₂@reduced graphene oxide nanotubes

Abstract

Ultra thin TiO₂ layer (2 nm)-coated SnO₂ nanotubes (NTs) wrapped by reduced graphene oxide (rGO) sheets (SnO₂@TiO₂@rGO) were synthesized as high capacity anode materials for lithium-ion batteries. The rationally designed anodes exhibited superior rate capability while maintaining a high discharge capacity of over 840 mA h g⁻¹ at a current density of 500 mA g⁻¹ after 50 cycles.

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As both the next-generation renewable energy sources and highly efficient energy storage devices, lithium-ion batteries (LIBs) have been commercialized to some extent in electric vehicles, cell phones, and other storage devices.^1–4 Nevertheless, conventional LIBs with graphite anodes have limitations in large theoretical capacity (372 mA h g⁻¹), desired for large-scale electric grids and energy storage systems.^5–8 As for the LIBs with higher energy density, metal oxides have been suggested as the promising anode candidates, due to their inexpensive costs, high theoretical capacity, and safety characteristics.^9–11 Among them, tin(IV) oxide (SnO₂) is one of the promising anode materials for LIBs, due to its higher theoretical capacity (782 mA h g⁻¹), enhanced battery safety, and good chemical stability with electrolytes.^12–16

However, several drawbacks exist for SnO₂ anodes in LIBs including: (i) severe volume expansion (>300%) during cycling which leads to poor cycling retention; (ii) formation of unstable solid electrolyte interphase (SEI) layer; and (iii) low electrical conductivity.^17,18 In order to resolve these issues, numerous attempts have been made.^19–22 For instance, different researches on Sn–C composite materials have been initially devised and suggested, and they were aimed at utilizing carbon to increase the low electrical conductivity of SnO₂ and Sn.^23–25 With this approach, however, increasing loading amount of carbon results in decreasing loading amount of either Sn or SnO₂, which lead to lower capacity. Another approach has been made to utilize titanium dioxide (TiO₂) with tin oxide to make a composite anode material. Combining with TiO₂, it forms stable solid-electrolyte interphase (SEI) layer, thereby contributing to more stable nanostructure that maintains structural integrity during lithiation/de-lithiation, with better cycle retention for longer cycles.^26,27 Different research strategies have been suggested such as TiO₂ nanotubes@SnO₂ nanoflakes, TiO₂/SnO₂/carbon hybrid nanofiber using thermal pyrolysis, SnO₂@TiO₂ double-shell nanotubes, and SnO₂@TiO₂ nanowires through atomic layer deposition (ALD).^27–30 Nevertheless, the limitation for these approaches is that general synthetic methods were rather complicated series of steps and not applicable for large-scale synthesis.

In this research, we have successfully synthesized ultra thin TiO₂ layer (2 nm)-coated SnO₂ NTs (hereafter, SnO₂@TiO₂ NTs) via electrospinning method, subsequent calcination process, and sol–gel process. With uniform TiO₂ coating layer on SnO₂ NTs, it forms a stable SEI layer, which then enhances cycle retention and structural integrity. In addition, upon reduced graphene oxide (rGO) wrapping, rGO-wrapped SnO₂@TiO₂ NTs (SnO₂@TiO₂@rGO NTs) were formed, where the electrical conductivity was significantly increased. With facile and simple synthetic sol–gel methods distinct from the previous works on SnO₂/TiO₂, it forms stable SEI layer, resulting in better cycle retention. Subsequently, rate capability is also improved by rGO wrapping on SnO₂@TiO₂ NTs.

The synthesis of SnO₂@TiO₂@rGO NTs was prepared via electrospinning process, calcination treatment, sol–gel process, and solution-based reduced graphene oxide (rGO)-wrapping process, as illustrated in Fig. 1. Using single-nozzle electrospinning process, Sn precursor/poly(vinylpyrrolidone) (PVP) composite nanofibers (NFs) were fabricated. Upon calcination process at 600 °C for 1 h and 700 °C for 1 h, SnO₂ NTs were formed by Kirkendall effect in the following steps:³¹ (i) as temperature increased, PVP was decomposed and tin (Sn) precursor on the outer surface of the fibers was oxidized and became SnO₂; (ii) Sn precursor at the core is also subsequently oxidized and with the Ostwald ripening allows the oxidized Sn at the core to move to the outer SnO₂ with larger grain sizes, in addition to the fact that CO₂ gas and H₂O gas from decomposition of PVP also moves Sn precursor at the core to the outer side; (iii) at the same time, inward flow of vacancies moved to the core at the place where Sn precursor moved outward; (iv) finally, upon subsequent temperature increase, SnO₂ underwent nanograin formation, growth, and reorganization, as shown in the previous literature.³²

Fig. 1 Schematic illustration on the formation process of SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs.

Surface morphology of as-spun Sn precursor/PVP NFs, SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs is shown in Fig. 2. The scanning electron microscopy (SEM) image of Sn precursor/PVP composite NFs with diameter ranged between 300 and 500 nm, is shown in Fig. 2a. The Sn precursor/PVP composite NFs were converted to SnO₂ NTs consisting of SnO₂ nanograins that have mean grain sizes of 30.6 nm calculated from Scherrer's eqn (1) for the (110) peak in Fig. S1† after calcination (Fig. 2b):

where τ is crystallite size of SnO₂, K is a dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity, and θ is the Bragg angle. Subsequently, TiO₂ layer was adopted on the surface of the SnO₂ NTs via sol–gel process. Fig. 2c shows the SEM image of SnO₂@TiO₂ NTs, with smoother surface morphology of shells than that of SnO₂ NTs. The hollow structure of as-synthesized samples is well maintained during sol–gel process of TiO₂, with surface area of 7.76 m² g⁻¹, as shown in Fig. S2.†

Fig. 2 SEM images of (a) electrospun Sn precursor/PVP NFs, (b) SnO₂ NTs, and (c) SnO₂@TiO₂ NTs. (d) TEM image of SnO₂@TiO₂ NT. (e) Magnified TEM image of the red frame in (d), which indicates TiO₂-coated outer layer of SnO₂@TiO₂ NT. (f) TEM elemental mapping images of SnO₂@TiO₂ NT according to each element (O, Ti, and Sn). (g) SAED pattern of SnO₂@TiO₂ NTs. TEM images of (h) SnO₂@TiO₂@rGO NT showing the wrapped rGO layers and (i) SnO₂@TiO₂@rGO NT having lattice spacing of SnO₂.

To clearly analyze the surface morphology and crystal structure, we carried out transmission electron microscopy (TEM) analysis. Fig. 2d shows the overall morphology of SnO₂@TiO₂ NT with uniform thickness across different areas. It can be shown from Fig. 2e that about 2 nm (solid yellow line) of TiO₂ layer was uniformly present on the surface of SnO₂. To illustrate the spatial distribution of SnO₂ and TiO₂, transmission electron microscopy energy-dispersive X-ray spectroscopy (TEM-EDS) elemental mapping was performed on as-synthesized SnO₂@TiO₂ NT (Fig. 2f). According to the elemental mapping images of SnO₂@TiO₂ NT, TiO₂ layers were conformally coated on the surface of SnO₂ nanograins. In the selected area electron diffraction (SAED) pattern of SnO₂@TiO₂ NT (Fig. 2g), however, crystal structure of TiO₂ was not clearly shown, as it can be speculated that the concentration of TiO₂ was too minimal to have enough intensity to have a clear diffraction pattern, as shown from previous literature.³⁰

To clearly confirm the crystal structure of TiO₂, another sample with five times higher concentration of Ti precursor (denoted as SnO₂@TiO₂ (×5)) was prepared. Fig. S3a† shows the TEM images of SnO₂@TiO₂ (×5) NT and Fig. S3b† shows its magnified image of red box. The marked interplanar d spacing of 0.352 nm corresponds to the (101) lattice plane of anatase TiO₂. The SAED pattern of SnO₂@TiO₂ (×5) sample indicates the polycrystalline nature of the as-synthesized sample and the diffraction rings can be readily assigned to the SnO₂ and dominant peaks of anatase TiO₂ such as (101), (004), and (204), which were shown in Fig. S3c.† This anatase structure was also confirmed through X-ray diffraction (XRD) pattern of SnO₂@TiO₂ (×5) (Fig. S3d†), where anatase peaks were clearly apparent in the lattice planes of (101) and (200) at 25.3° and 48.0°.^33,34

Upon rGO wrapping, rGO layers were coated on TiO₂ layer and TEM image of SnO₂@TiO₂@rGO is shown in Fig. 2h, where the thin layers of rGO are shown on the outer side of SnO₂@TiO₂ NT. Lattice spacing of SnO₂ in SnO₂@TiO₂@rGO NT was shown in Fig. 2i, which was also in agreement with the XRD pattern of SnO₂ NTs.^35–37 Based on thermogravimetric analysis (TGA) in air of the SnO₂@TiO₂@rGO NTs, it was calculated that the rGO content was ca. 3.10 wt% in the as-synthesized sample (Fig. 3a).^38,39

Fig. 3 (a) TGA curve and (b) Raman spectra of SnO₂@TiO₂@rGO.

Fig. 3b shows the Raman spectrum of SnO₂@TiO₂@rGO NTs. Two peaks at 1343 cm⁻¹ and 1599 cm⁻¹, which corresponded to the D and G band of characteristic carbon peaks, were found and the dimensional ratio of the D band to the G band for the sample was estimated to be 1.09, which indicates the formation of amorphous carbon which are in good agreement with Raman peaks of rGO in similar designs of materials.^40–42

Surface chemical states of SnO₂@TiO₂ and SnO₂@TiO₂@rGO NTs were also analyzed using X-ray photoelectron spectroscopy (XPS). XPS spectra of Sn and Ti in SnO₂@TiO₂ NTs are presented in Fig. 4a and b, respectively. Sn⁴⁺ 3d_3/2 and 3d_5/2 have binding energy peaks at 495.0 and 486.6 eV, which are similar to the previously reported values.²⁸ Ti⁴⁺ 2p_1/2 and 2p_3/2 have binding energy peaks at 464.5 and 458.8 eV, which are also similar to the previously reported values.^28,43 XPS spectra of Sn and Ti in SnO₂@TiO₂@rGO NTs are presented in Fig. 4c and d, respectively. Sn⁴⁺ 3d_3/2 and 3d_5/2 have binding energy peaks at 495.5 and 487.1 eV, which are similar to the previously reported values.²⁸ Ti⁴⁺ 2p_1/2 and 2p_3/2 have binding energy peaks at 465.0 and 459.3 eV, which are also similar to the previously reported values.^28,44 In summary, XPS spectra for SnO₂@TiO₂ and SnO₂@TiO₂@rGO NTs confirmed that SnO₂ and TiO₂ are present.

Fig. 4 XPS spectrum of (a) Sn, (b) Ti in SnO₂@TiO₂ NTs and (c) Sn, (d) Ti in SnO₂@TiO₂@rGO NTs.

A series of electrochemical characterizations were carried out to investigate the electrochemical properties of the SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs based electrodes using 2032-type coin cells. The detailed cell fabrication procedure was described in ESI.† Fig. S4† shows cyclic voltammogram (CV) curves of the first, second and third cycles of SnO₂@TiO₂ NTs within the range of 0.01–3.0 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹. Typically, it is widely known that electrochemical reaction of SnO₂ involves two steps (reduction of SnO₂ and alloying and de-alloying of Sn) and Li insertion/de-insertion reaction of TiO₂ is an intercalation reaction, as shown in the reactions below:^30,45,46

SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O (reduction of SnO₂)

xLi⁺ + xe⁻ + Sn ↔ Li_xSn (0 ≤ x ≤ 4.4) (alloying/de-alloying of Sn)

TiO₂ + xLi⁺ + xe⁻ ↔ Li_xTiO₂ (Li insertion/deinsertion of TiO₂)

In the first cycle, a cathodic peak of 0.75 V was observed in the discharge process, which can be ascribed to the reduction of SnO₂ to Sn and the formation of Li₂O as well as formation of SEI layer, in accordance with the previous literature.^45,52 This process usually occurs at a voltage higher than 0.6 V and is generally known to be irreversible.^47–50 Second cathode peak of 0.3 V was observed in the discharge process, which accounts for the alloying of Sn to Li_xSn. Alloying process occurs at a lower voltage and is known to be reversible.⁵¹ With the formation of Li₂O and SEI layer in the 1st cycle, the general reaction becomes reversible after the 1st cycle, where CV curves do not vary significantly from the 2nd to 3rd cycle in the discharge process, indicating that the reaction has proceeded with the alloying reaction. At the same time, small peaks at 1.0 V and 1.5 V are mainly attributed to the reaction between Li and TiO₂, which is broadly coated on the surface of SnO₂.⁵³ This concurs with the previous study that initial CV curve of TiO₂ starts with −0.4 mA at 1.0 V, and shows one broad peak at 1.5 V.⁵² From the 2nd to 3rd cycle in the charge process, two peaks at 0.6 V and 1.3 V are related to the de-alloying of Li_xSn and oxidation of Sn to SnO₂ and it is suggested that small peak at 1.75 V is mainly due to the single-phase Li-insertion/de-insertion reaction in TiO₂.⁵² Presence of smooth, small CV peaks for TiO₂ is in agreement with the previous observation that nano-sized systems result in smaller, broader peaks rather than sharp peaks.⁵⁴

To examine the initial irreversible capacity loss as a result of conversion reaction (reduction of SnO₂), the first charge and discharge profiles of SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs were analyzed in the voltage range of 0.005–3.0 V at a current density of 50 mA g⁻¹, as shown in Fig. 5a. SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs have initial discharge capacity of 1429.2 mA h g⁻¹, 1312.5 mA h g⁻¹, and 1531.7 mA h g⁻¹ and initial charge capacity of 944.3 mA h g⁻¹, 873.8 mA h g⁻¹, and 1029.2 mA h g⁻¹, which correspond to the initial coulombic efficiency of 66.1%, 66.6%, and 67.2% for SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs, with similar efficiencies. The capacity values are higher than the theoretical capacity of SnO₂ (782 mA h g⁻¹) and theoretical capacity of SnO₂@TiO₂@rGO (C_{SnO2@TiO2@rGO} = C_{SnO2*% SnO2 mass} + C_{rGO*% rGO mass} = (782) × (0.969) + (372) × (0.031) = 769.3 mA h g⁻¹), which is attributed to the synergistic effects of different components.

Fig. 5 Electrochemical performances of SnO₂@TiO₂@rGO, SnO₂@TiO₂, and SnO₂ NTs. (a) Galvanostatic first cycle discharge/charge curves at a current density of 50 mA g⁻¹ between 0.005 V and 3.0 V. (b) Cycle performance of various samples at a current density of 500 mA g⁻¹ between 0.01 V and 3.0 V. (c) Variation in the discharge/charge profiles of SnO₂@TiO₂@rGO during cycling at a current density of 500 mA g⁻¹. (d) Rate capabilities of various samples. The same discharge/charge rate was used from 100–5000 mA g⁻¹ between 0.01 V and 3.0 V.

To understand the effect of coated TiO₂ and wrapped rGO layer on cycle retention, SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs were cycled at a current density of 500 mA g⁻¹ for 70 cycles, as presented in Fig. 5b. Electrochemical properties of SnO₂ NTs made under different calcination temperatures were tested, and it was shown that the condition for calcination described above was the optimal condition, as shown in the Fig. S5.† Comparing the cycle retention of SnO₂ NTs, SnO₂@TiO₂, and SnO₂@TiO₂@rGO from the 2nd cycle to 70th cycle, it is evident that SnO₂@TiO₂@rGO exhibits better cycle retention (79.7%) compared with that of pristine SnO₂ (20.3%) and SnO₂@TiO₂ (67.1%) NTs. Coulombic efficiency of SnO₂@TiO₂@rGO NTs maintains a stable state up to 70 cycles, which suggests that enhanced capacity retention is mainly attributed to the hybrid structure that forms stable SEI layer by coating TiO₂ layer and increases electrical conductivity by wrapping rGO layers. In addition, the charge and discharge profile of SnO₂@TiO₂@rGO and SnO₂@TiO₂ NTs at a current density of 500 mA g⁻¹ are shown in Fig. 5c and S6.†

In order to demonstrate that thin TiO₂ layer on the surface of SnO₂ helps to form more thin and stable SEI layer, we conducted ex situ SEM analysis of electrodes after 50 cycles at 500 mA g⁻¹. The thinner and smoother SEI layer was formed on the surface of SnO₂@TiO₂ after 50 cycles at 500 mA g⁻¹ than that of SnO₂ in the SEM images (Fig. S7a and b†). Furthermore, we observed that thinner and smoother SEI layer was formed on the surface of SnO₂@TiO₂@rGO than both SnO₂ and SnO₂@TiO₂ (Fig. S7c†). Owing to the formation of thin and stable SEI layer on the surface, SnO₂@TiO₂@rGO shows much improved cycle retention and reversible capacity.

The initial irreversible capacity of both SnO₂@TiO₂ and SnO₂@TiO₂@rGO NTs comes from the formation of Li₂O, an amorphous matrix, and SEI layer.^47,48 After the 2nd cycle, both the discharge and charge capacity of SnO₂@TiO₂ NTs decreased from 812.7 mA h g⁻¹ to 655.3 mA h g⁻¹ and from 769.4 mA h g⁻¹ to 642.6 mA h g⁻¹, showing capacity retention less than 85%. On the other hand, after the 2nd cycle, both the discharge and charge capacity of SnO₂@TiO₂@rGO NTs only slightly decreased from 974.8 mA h g⁻¹ to 843.3 mA h g⁻¹ and from 930.8 mA h g⁻¹ to 825.8 mA h g⁻¹, maintaining capacity above 800 mA h g⁻¹ with better capacity retention even after 50 cycles.

To understand the effect of rGO layer in enhancement of electrical conductivity, we evaluate the rate capabilities of SnO₂, SnO₂@TiO₂, and SnO₂@TiO₂@rGO NTs at various current densities (Fig. 5d). SnO₂@TiO₂@rGO NTs exhibited better rate capabilities compared with both SnO₂@TiO₂ and SnO₂ NTs, especially at higher current densities, which can be suggested from the improved electrical conductivity with rGO-wrapping process that can be demonstrated by impedance tests. Fig. S8† shows the Nyquist plot of each sample after 50th cycle. Semicircles in different frequency regions are ascribed to different factors: semicircle in high frequency region is attributed to the contact resistance by SEI layer formation; semicircle in middle frequency region is attributed to the charge transfer resistance; the straight line in low frequency region describes the mass transfer of lithium-ions.^55,56 After the 50th cycle, it is clearly shown that the charge-transfer resistance (R_ct) of SnO₂@TiO₂@rGO is much smaller compared with those of SnO₂@TiO₂ and SnO₂ NTs. Such trends of semicircle areas in Nyquist plots can be suggested that SnO₂@TiO₂@rGO, upon rGO-wrapping, greatly improved the electrical conductivity of SnO₂@TiO₂, maintained even after 50 cycles.^57,58

Conclusions

In this work, we successfully synthesized a new hybrid structure of SnO₂@TiO₂@rGO NTs. The anode electrode with SnO₂@TiO₂@rGO NTs exhibited excellent cycle retention and good rate capabilities compared with SnO₂@TiO₂ and SnO₂ NTs. Such outstanding features are attributed to the following reasons: (i) with electrospinning and subsequent calcination, SnO₂ NTs were formed, which maintain structural integrity and allow shorter Li-ion diffusion path; (ii) sol–gel process of TiO₂ on SnO₂ NTs allows TiO₂ coated layer to act as a surface protection layer which provides stable SEI layer; (iii) upon rGO-wrapping, electrical conductivity was improved, which facilitates charge transport to the active material, leading to superior rate capability at higher c-rates. Our suggested strategy that incorporates both thin TiO₂ overlayer and rGO sheets can be applied to other anode materials, such as Si, to both enhance the cycle retention and rate capability.

Acknowledgements

This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (No. NRF-2014M1A8A1049303) and End-Run grant from KAIST funded by the Korea government in 2015 (Ministry of Science ICT & Future Planning) (N01150615).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23704a

‡ These authors contributed equally to this work.

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