Synergistic effect of graphene and polypyrrole to enhance the SnO₂ anode performance in lithium-ion batteries

Abstract

In this work, a synergistic effect of reduced graphene oxide (rGO) and polypyrrole (PPy) was studied in terms of their promotional role to enhance the capacity and cyclic stability of hollow SnO₂ anodes in lithium-ion batteries. The core–shell structured hollow SnO₂/rGO/PPy nanocomposites were synthesized using a hydrothermal method followed by an in situ chemical-polymerization route. Substantially improved cycling stability and rate capabilities are achieved on the SnO₂/rGO/PPy ternary anodes. The exceptional cycling performance is due to the hollow ternary core–shell structure covered with PPy buffer layers along with the graphene frameworks further benefiting Li⁺ diffusivity and electrical conductivity. The significantly increased Li⁺ diffusion coefficient improves rate performance and the large current charge and discharge. Thus, taking all of these benefits together including the hollow structures of SnO₂ particles, role of the buffer of PPy, and effective matrix of graphene, the ternary nanocomposites yield a robust architecture for anode materials in high-performance Li-ion batteries.

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Introduction

SnO₂-based materials have become some of the most promising anode materials for next generation Li-ion batteries (LIBs), due to their high capacity, low cost, good safety, and low toxicity.^1–5 The theoretical specific capacity of SnO₂ is 782 mA h g⁻¹, which is much higher than the currently used graphite anode materials (372 mA h g⁻¹). Furthermore, they have high energy density due to the low potentials of the lithium alloying/de-alloying reactions with SnO₂.⁶ However, the huge volume expansion of SnO₂ particles during charge–discharge cycles results in rapid capacity degradation,⁷ significantly limiting its practical application.

To address this issue, nanostructured SnO₂ materials^8–10 and unique SnO₂ morphologies, such as SnO₂ nanorod arrays,² nanotubes,¹¹ and hollow nanostructures^12–15 have been extensively studied with an aim to improving cycling stability by shortening lithium diffusion length and reducing internal strain when using SnO₂ for anodes. In addition, conducting polymers have been introduced into anode materials for enhancing the LIB performance.^16–19 Apart from improving conductivity^20–24 the soft polymer matrix also can relax the internal stress of solid particle anodes that suffer from severe volume change during charge–discharge.²⁵ Recently, SnO₂/polypyrrole (PPy) composites, such as SnO₂@polypyrrole core–shell structures,^25,26 SnO₂@polypyrrole hybrid nanowires²⁷ and hollow SnO₂@polypyrrole nanocomposites,²⁸ have demonstrated vastly improved cycling performance compared to the standalone SnO₂ nanoparticles. In these composites, the polypyrrole matrix effectively prevented the agglomeration of the SnO₂ nanoparticles and elastically buffered the volumetric change in the nanoparticles.

Alternatively, SnO₂ composites containing a buffer layer, such as carbon materials, exhibited enhanced stability due to their good overall electronic conductivity and their capability to mitigate the internal stress of SnO₂.^29–34 Especially, graphene's flexibility, large surface area (up to 2630 m² g⁻¹), and chemical stability, combined with its excellent electrical and thermal conductivity, make it promising as a catalyst in fuel and dye-sensitized solar cells. Chemically functionalized graphene can also improve storage and diffusion of ionic species and electric charge in batteries and supercapacitors.³⁵ For these reasons, it holds great promise as an advanced material for energy storage technologies.^36–39 It is worth noting in particular, graphene nanoflakes,⁴⁰ obtained from the exfoliation of pristine graphite, represent an ideal yet not fully explored material platform for battery electrodes. It have exhibited excellent electrochemical performances because of its high crystallinity and small lateral size (<100 nm) for assuring fast electron transport to the electrode support and Li⁺ ions storage, providing much stronger (up to 50%) binding energies for Li⁺ and decreased energy barriers for Li diffusion. To prepare graphene nanoflake ink, the liquid phase surfactant-assisted exfoliation in water allows the facile preparation of large-scale, homogeneous graphene dispersions of varying concentrations, showing great advantages over most other exfoliation routes that often involve the oxidation and exfoliation of powdered graphite to yield graphene oxide, and subsequent reduction steps either by high-temperature thermal annealing or chemical methods.^41,42 Unfortunately, single graphene anodes experience more than 50% irreversible capacity loss during a short-term charge/discharge cycling (∼50 cycles). Solid nanoparticles are required to incorporate among these graphene layers to alleviate the re-stacking problem.^43–45

Despite significant progress in alleviating the volume expansion of SnO₂ by integrating conducting polymer or graphene to build binary systems, there is still ample opportunity to further improve the electrochemical performances of SnO₂ anodes. Although there is a graphene/SnO₂/PPy ternary composite applied to the supercapacitor,⁴⁶ which is composed of a thin conducting film of PPy on the surface of graphene/SnO₂, the SnO₂ morphology in the composite is solid nanoparticles, there is still room to further improve the electrochemical performances of SnO₂ anodes if it was applied in the Li-ion batteries. In this work, a novel SnO₂/graphene/polypyrrole ternary nanocomposites consisting of hollow SnO₂ particles simultaneously surrounded by PPy and graphene was prepared for the first time on the basis of our previously reported core–shell structured hollow SnO₂ anode.²⁸ Besides the likely synergistic effect between the hollow SnO₂ core–shell structure and PPy buffer layers, the two-dimensional graphene not only enhances the strength of the electrode, but it also effectively improves electron and Li-transport in the electrode. The exceptional cycling stability and much improved rate performance were achieved on the ternary nanocomposite anode, when compared to single SnO₂ and binary SnO₂/PPy anode systems.

Experimental

Materials preparation

Core–shell structured hollow SnO₂/graphene/polypyrrole ternary nanocomposites were prepared via a hydrothermal method followed by an in situ chemical-polymerization route. First, hollow SnO₂ microspheres were prepared by a hydrothermal method we reported before.⁴⁷ In brief, CO(NH₂)₂ (1.92 g) and Na₂SnO₃·3H₂O (1.28 g) were dissolved in a 320 mL mixed solution of water and ethanol (EtOH/H₂O = 0.6, v/v). Then, the solution was transferred into a Teflon-lined stainless steel autoclave at a temperature of 180 °C for 24 h. After cooling down to room temperature, the precipitate was filtered, washed with deionized water and ethanol, and dried at 90 °C overnight. The resulting hollow SnO₂ particles were calcined at 500 °C in atmosphere environment for one hour. Graphene oxide (GO) used in this work was synthesized through a modified Hummers method.^48,49 Reduction of GO was accomplished using a high temperature treatment at 800 °C for 30 min under Ar atmosphere. Then, the core–shell structured ternary nanocomposites were prepared via an in situ chemical-polymerization route. In a typical procedure, as shown in Fig. 1, 0.1 g of hollow SnO₂ microspheres were mixed with a 40 mL aqueous solution containing 4 mg of sodium lauryl sulfate (SDS) and 0.02 g reduced graphene oxide (rGO) followed by sonication for 30 min and magnetic stirring for 3 h. After adding a 25.8 μL pyrrole monomer, the solution was continuously stirred for another 1 h. Then, 11.2 mL of 0.1 M ammonium persulfate aqueous solution was dropwise added into the above solution. The color of solution is gradually changing from light grey to black indicative of a formation of PPy that adhered to the surface of graphene and hollow SnO₂ microspheres. The polymerization process was conducted while stirring for 4 h at room temperature. The resulting black composites were centrifuged, washed with deionized water and ethanol at least three times, and then dried in a vacuum oven at 80 °C overnight. As comparison, the synthesis procedure of SnO₂/PPy binary composites was similar with the synthesis procedure of ternary nanocomposites, but without adding reduced graphene oxide in the procedure.²⁸

Fig. 1 Scheme of synthesis for core–shell structured hollow SnO₂/rGO/polypyrrole ternary nanocomposites.

Materials characterization

The prepared SnO₂/rGO/PPy ternary nanocomposites were extensively characterized using X-ray diffraction (XRD, Rigaku D/MAX-RB), field emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron microscopy (TEM, FEI TECNAI G2), FT-IR (EQUINOX55), and TGA (STA-400).

Electrochemical measurements

As for LIB tests, the nanocomposites were used for fabricating the working electrodes consisting of 80 wt% SnO₂/rGO/PPy nanocomposites, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF). A piece of lithium foil was used for the combined counter and reference electrode. Celgard 2400 was used as a separator film. 1.0 M LiPF₆ dissolved in a 1 : 1 volume ratio mixture of ethylenecarbonate (EC) and dimethyl carbonate (DMC) was used as the electrolyte. The amount of electrolyte was much more than needed. All cells were assembled in an argon-filled glove box. The fabricated coin cell (CR2032) with ca. 2 mg active species (SnO₂ and rGO) underwent galvanostatic charge–discharge cycling and cyclic voltammetry (CV) testing using a battery testing system (Arbin BT-2000) and an electrochemical workstation CHI660B, respectively. Electrochemical tests were carried out at room temperature. The specific capacity was calculated based on the total weight of SnO₂ and rGO. The charge–discharge tests were conducted at a rate of 0.1C (1C = 782 mA g⁻¹) in a voltage window of 0.04–3.00 V (vs. Li/Li⁺). The CV curves were measured in a potential range of 0.02–2.70 V (vs. Li/Li⁺) at a scan rate of 0.5 mV s⁻¹.

Results and discussion

Structures and morphologies

Fig. 2 shows the XRD patterns of hollow SnO₂/rGO/PPy ternary nanocomposites. The diffraction peaks observed are well indexed to the tetragonal rutile structure of cassiterite SnO₂ (JCPDS no. 41-1445). It is noted that there is no obvious rGO patterns in the nanocomposites, due to the overlap of the diffraction peak for rGO (002) facet with that of (110) facet for SnO₂ and also the relatively weak intensity compared with SnO₂. Besides, it is also due to the low content of rGO in the nanocomposites as determined by TGA in the next text.

Fig. 2 XRD patterns of hollow SnO₂/rGO/PPy core–shell nanocomposites.

As shown in Fig. 3, the FT-IR spectra of SnO₂/rGO/PPy ternary nanocomposites were compared to pristine SnO₂ hollow microspheres, standalone PPy, and rGO. The bands in the range of 537–623 cm⁻¹ observed with the SnO₂ hollow microspheres can be assigned to the anti-symmetric and symmetric vibrations of Sn–O–Sn.^50,51 The bands centered at 1707 and 1596 cm⁻¹ for the PPy correspond to typical C=C in plane vibration. In addition, the characteristic bands of PPy for C–C and C–H ring stretching were found at 1400 and 1258 cm⁻¹, respectively. The sharp peak located at 1049 cm⁻¹ is attributed to the in-plane vibrations of C–H. The band at 929 cm⁻¹ can be assigned to N–H in-plane vibrations.^52,53 The band at about 1580 cm⁻¹ is related to rGO, which can be attributed to the skeletal vibration of the graphene sheets or the remaining sp² character.^54–56 The peak present at about 1230 cm⁻¹ should be attributed to the C–O–C vibration, while the peak at 1730 cm⁻¹ should be caused by C=O group.^57,58 Similar to these individual components, the FT-IR spectra of SnO₂/rGO/PPy ternary nanocomposites have the main characteristic peaks of pristine SnO₂ hollow microspheres, standalone PPy and rGO, respectively, albeit with relatively low intensities. Thus, the FT-IR analysis ascertain the simultaneous existence of SnO₂, PPy and rGO in the ternary nanocomposites.

Fig. 3 FT-IR spectra of hollow SnO₂/rGO/PPy core–shell nanocomposites.

In order to determine the content of each component in the ternary composite, thermogravimetric analysis (TGA) was carried out for a binary SnO₂/PPy and a ternary SnO₂/rGO/PPy sample and shown in Fig. 4. Both composite samples were heated from 25 to 800 °C in air atmosphere. A mass loss of 21% was observed for the SnO₂/PPy binary composite at a temperature of 550 °C, indicating a PPy content of 21 wt%. Meanwhile, at a temperature of 672 °C, the total mass loss of ternary nanocomposites was determined to be 38 wt%, which includes the content of both PPy and rGO. Thus, contents of PPy and rGO in the ternary composite are calculated to be 16.5 and 21.5 wt%, respectively. The slopes of curves of the lower temperature zone (from room temperature to about 240 °C) and higher temperature zone (from 240 °C to about 650 °C) are different, the slight weight loss at lower temperature zone is attributed to the elimination of the moisture in the samples,²² the major weight loss at higher temperature zone of all the samples is due to the degradation of PPy and rGO.

Fig. 4 TGA curves of hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites, hollow SnO₂/21 wt% PPy nanocomposites, pure SnO₂ and PPy.

The morphology of binary SnO₂/PPy and the SnO₂/rGO/PPy ternary nanocomposites were further examined by SEM, as shown in Fig. 5. In the binary SnO₂/PPy composites, SnO₂ particles with a size range between 100–200 nm were uniformly covered by polymerized PPy thin layers. After introducing rGO into the ternary systems, some SnO₂ microspheres adhere to the external of laminated nanocomposites, and some are coated by laminated nanocomposites. The SnO₂ microspheres and laminated nanocomposites are intertwined together, forming a three-dimensional network structure. The size of SnO₂ microspheres is relatively uniform, and the average diameter is around 500 nm. Because both rGO and PPy layers are intrinsically plastic and soft, polymerized PPy layers are grown irregularly on the irregular surface of graphene matrix. Thus, there are no obvious boundaries between the graphene and PPy layers and it is difficult to distinguish them according to the SEM images.

Fig. 5 (a and b) SEM images for the binary SnO₂/21 wt% PPy; (c and d) SEM images for the core–shell structured hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy nanocomposites at different magnification.

Meanwhile, Fig. 6 shows the TEM images of these binary and ternary composites. Fig. 6a clearly confirmed the unique hollow feature of SnO₂ particles. The diameter of the hole inside particles is around 100 nm and the thickness of hollow SnO₂ layers is around 20 nm. A core–shell structure of SnO₂/PPy was observed in Fig. 6b, in which hollow SnO₂ particles with diameters of 100–200 nm were covered by PPy layers with a thickness of 50 nm. In the ternary composite (Fig. 6c and d), SnO₂ particles were dispersed into rGO and PPy layers. Some hollow SnO₂ particles are broken, because of sonication in the preparation process of ternary nanocomposites. Fig. 6e and f presents the HRTEM images of ternary nanocomposites, some scattered SnO₂ particles are observed in the laminated structure, and lattice fringe with an interplanar spacing of about 0.335 nm that correspond to the (110) planes of SnO₂ is further identified. Moreover, the graphene appears to possess a multi-layer stack morphology, and the interplanar spacing is about 0.36 nm. This value is much larger than that of pure graphite (0.335 nm), which indicates graphene-like layered structures resulted from the exfoliation of graphite by formation of oxygen-containing groups. Amorphous PPy layers coated on the graphene and SnO₂ particles can also be observed in the micrographs and are marked by white arrows.

Fig. 6 Typical TEM images for the hollow SnO₂ microspheres (a), core–shell structured hollow SnO₂/21 wt% PPy nanocomposites (b), and core–shell structured hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy nanocomposites at different magnification (c–f).

Battery tests and electrochemical properties

Fig. 7a shows the cycling performance of core–shell structured hollow SnO₂/rGO/PPy ternary nanocomposites at 0.1C in the voltage window of 0.04–3.00 V, in comparison with those of hollow SnO₂ microspheres and core–shell structured hollow SnO₂/21 wt% PPy nanocomposites anodes. The retained specific capacity for the hollow SnO₂ microspheres anode is 406.5 mA h g⁻¹ after the 65th cycle, exhibiting insufficient cyclic stability. An enhancement was observed with the binary SnO₂/21 wt% PPy anode, evidenced by a retained capacity of 448.4 mA h g⁻¹ after 100 cycles. A substantial improvement was achieved on the ternary SnO₂/rGO/PPy nanocomposite anode, demonstrating a reversible capacity of 647.8 mA h g⁻¹ after the 100th cycle. Noteworthy, the first 10 cycles of capacities are higher than the theoretical reversible capacity of SnO₂ (782 mA h g⁻¹). Apart from the partially reversible reaction as described in eqn SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O, the hollow structure is able to provide more interfacial area for de-embedding lithium. In addition, because graphene can adsorb lithium ions on both sides, it holds substantially higher specific capacity than that of graphite. Meanwhile, the single layer of graphene provides a facile route for the diffusion of Li⁺, since the space for lithium intercalation is much larger than that in graphite interlayers, and thus more lithium species are reversibly inserted into the SnO₂/rGO/PPy ternary composite materials. Importantly, the cycling performance of core–shell structured hollow SnO₂/rGO/PPy ternary nanocomposites is significantly enhanced after integrating PPy and rGO simultaneously with the hollow SnO₂ microsphere anodes.⁴⁷ The newly developed ternary nanocomposite anode is also superior to other studied SnO₂-based anodes, including SnO₂ nanoparticles–PPy composites,²⁵ SnO₂ nano-single crystals,⁸ SnO₂ hollow nanotubes,⁵⁹ mesoporous SnO₂ on multiwalled carbon nanotubes (MWCNTs),⁶⁰ graphene nanosheets,³⁷ and SnO₂/graphene nanocomposites.⁶¹ Meanwhile, the coulombic efficiency of core–shell structured hollow SnO₂/rGO/PPy ternary nanocomposites anode for the first cycle is only 58.2%, but it is raised to 90.6% for the 2nd cycle and up to 99% during subsequent long-term cycles. Also, compared to the hollow SnO₂ microsphere anode and hollow SnO₂–21 wt% PPy core–shell nanocomposite anode, the coulombic efficiency of the ternary nanocomposites is obviously improved.

Fig. 7 The cycling performance (a) and rate capability (b) for hollow SnO₂ microspheres, core–shell structured hollow SnO₂/21 wt% PPy nanocomposites and core–shell structured hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy nanocomposites.

Fig. 7b exhibited rate capabilities of various anodes, which were studied by varying current densities from 78 mA g⁻¹ (0.1C) to 3900 mA g⁻¹ (5C). Overall, the rate capability of the ternary SnO₂/rGO/PPy nanocomposites was improved when compared to other anodes. For instance, the ternary anode is capable of delivering a substantial capacity above 1082.4, 755.7, 657.1, and 465.4 mA h g⁻¹ at each current density of 78 (0.1C), 390 (0.5C), 780 (1.0C), and 1560 (2.0C) mA g⁻¹, respectively. Even when the highest current density of 3900 mA g⁻¹ (5.0C) was applied, the ternary anode exhibited a reversible capacity above 117.6 mA h g⁻¹. It has a rapid capacity decay compared with the reversible capacity at lower current density. When charging and discharging at large current, Li⁺ intercalation/deintercalation reaction of active material will occur rapidly, causing rapid generation and decomposition of tin lithium alloys. So, the huge internal stress in the electrode is more easily to form in a short time compared with lower current density, can cause electrode pulverization, fall off, and failure. However, in the SnO₂/rGO/PPy nanocomposite anodes, the synergistic effect of rGO and PPy can effectively buffer huge volume change, improve the electrical contact. Noteworthy, when the charging–discharging rates were reduced back from high current to low current (0.1C) after 60 cycles, a specific discharge capacity of 924.2 mA h g⁻¹ was recovered, indicating that 68.03% of the initial reversible capacity (1358.6 mA h g⁻¹) was retained. Importantly, the determined rate capabilities from SnO₂/rGO/PPy ternary nanocomposites anode is better than those of other SnO₂-based nanocomposites.^14,28,47,61 These results demonstrate that the core–shell structured hollow SnO₂/rGO/PPy can tolerate huge volume change and provide sufficient charge (Li⁺ and electron) transport under high-rate operational conditions, which are desirable characteristics for high-power applications. The significantly enhanced cycling and rate performance are likely due to the unique configuration that consists of hollow SnO₂ microspheres surrounded by rGO and PPy layers simultaneously. The rGO and PPy work as effective separation and buffer layers during the lithium insertion/extraction process by preventing the agglomeration of particles, buffering the volume expansion, and maintaining the integrity of the electrode. Compared to the binary SnO₂/PPy nanocomposite anode, the rGO in SnO₂/rGO/PPy ternary nanocomposites has larger specific surface area and more flexible structure. Furthermore, the high conductivity of graphene facilitates electron transfer in the electrode, thereby reducing the electrochemical polarization and improving the high-rate performance.

AC impedance measurements were performed on the hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites electrode, hollow SnO₂/21 wt% PPy nanocomposites electrode and pure SnO₂ electrode. The electrodes were cycled galvanostatically for four cycles to ensure the stable formation of the SEI layers on the surface of the electrode. The AC impedance experiments were then performed at 0.4 V in the 4th charge cycle. As shown in Fig. 8, the Nyquist plot consists of a semicircle and a straight line, indicative of limitations in the charge transfer reaction (R_ct) and the diffusion of Li⁺ in the bulk electrode, respectively. The diameters of the semicircles for hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites electrode, hollow SnO₂/21 wt% PPy nanocomposites electrode and pure SnO₂ electrode are 93.9 Ω, 107.5 Ω and 162.8 Ω, respectively. The total resistance value of hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites electrode is smallest. The result confirms that the incorporation of rGO and PPy coating is an effective method for enhancing the electron transport of hollow SnO₂ microspheres, which leads to a significant improvement in the electrochemical performance.

Fig. 8 EIS for hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites, hollow SnO₂/21 wt% PPy nanocomposites and pure SnO₂.

To further investigate electrochemical reactivity of the SnO₂/rGO/PPy ternary nanocomposites, cyclic voltammetry was recorded in a potential range of 0.02–2.70 V (vs. Li/Li⁺) at a scan rate of 0.5 mV s⁻¹. As shown in Fig. 9, the initial CV curves of the SnO₂/rGO/PPy ternary nanocomposites are similar to those of hollow SnO₂ microsphere anodes,⁴⁷ suggesting that the PPy and rGO have no significant influence on the lithium insertion–extraction reactions. The peak at 0.60 V on the first cathodic scan is ascribed to the reduction of SnO₂ to Sn, as described in eqn (1), and the formation of Li₂O. Additionally, it is also associated with the formation of the solid electrolyte interface (SEI) layer,^14,62 resulting in a large irreversible capacity loss in the initial cycle. Subsequently, as described by eqn (2), the cathodic peak extending to 0.02 V and the anodic peak at 0.63 V are assigned to the alloying and dealloying of Sn and Li, respectively.⁷ In the anodic process, two oxidation peaks around 0.16 V and 0.61 V stand for the lithium extraction from the rGO (eqn (3)) and dealloying of the Li_xSn, respectively. In the meantime, an oxidation peak around 1.29 V and a reduction peak around 1.11 V are also clearly observed, which are most likely due to the partially reversible reaction of eqn (1),^1,63 corresponding to the formation and decomposition of nanosized Li₂O during subsequent cycles, because the Li–Sn alloying–dealloying reactions only occur below 1.0 V.

SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O (1)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) (2)

C(rGO) + xLi⁺ + xe⁻ ↔ Li_xC (3)

Fig. 9 The initial CV curves for the ternary SnO₂/rGO/PPy nanocomposite anode.

Fig. 10 shows the 1st, 2nd, 10th, 20th, 40th, 80th and 100th charge–discharge voltage profiles of the hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites at a current density of 78 mA g⁻¹ with a voltage range of 0.04–3.00 V. The initial small plateau in the potential range of 1.0 to 0.88 V corresponds to a irreversible conversion reaction between SnO₂ and Li⁺, resulting in the formation of Sn and Li₂O in the first discharge process, and also containing the formation of SEI layers and electrolyte decomposition. These reactions are identified as irreversible reactions. The plateau correspond with the peak around 0.60 V on the first cathodic scan in the CV curves. This is because these reactions form a wider peak due to their overlap, and its location has shifted in the cathodic scan process. The plateau (from 1.0 to 0.88 V) almost disappears at the second cycle, demonstrating that most Li₂O and SEI layers are formed in the first cycle. The following long slope profiles of the SnO₂/21.5 wt% rGO/16.5 wt% PPy nanocomposite indicate the formation of Li–Sn alloys. The first discharge capacity of the composites is 1144.6 mA h g⁻¹. The initial irreversible capacity is as high as 41.8%, which is likely caused by the formation of SEI layers, electrolyte decomposition, and the irreversible reaction of SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O during the discharge process. It is worth noting that, after 10 cycles, the reversible cycling capacity is well retained, suggesting an excellent reversibility of lithium insertion/extraction reactions in the nanocomposite.

Fig. 10 The typical voltage profiles for the hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites anode.

In order to elucidate the role of rGO during the lithium reactions, the initial CV curves of the ternary SnO₂/rGO/PPy anode was compared with those of binary SnO₂/PPy as displayed in Fig. 11. In the first cycle, the potential differences between oxidation and reduction peaks are 0.131 V and 0.032 V for the binary and ternary nanocomposites, respectively. The smaller peak potential difference of ternary nanocomposites suggests that the electrochemical reaction reversibility of the SnO₂/rGO/PPy anode is higher relative to graphene-free SnO₂/PPy anode.⁶⁴ Thus, the presence of rGO in the ternary anode improves the electrochemical reaction reversibility of the lithium insertion–extraction reactions. Likewise, in the second cycle as shown in Fig. 11b, SnO₂/rGO/PPy ternary nanocomposites have a better reversibility. These results are in good agreement of the improved cyclic stability measured with the SnO₂/rGO/PPy ternary nanocomposite anodes.

Fig. 11 The first (a) and second (b) CV curves for hollow SnO₂/21 wt% PPy core–shell nanocomposite and hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposite.

The Li⁺ diffusion coefficient was further determined as a function of anode component by varying the scan rates of CV curves from 0.1, 0.2, 0.3, 0.4 and 0.5 mV s⁻¹. The comparison among single SnO₂ microspheres, binary SnO₂/PPy, and ternary SnO₂/rGO/PPy is displayed in Fig. 12. The relationships of peak current (I_p) and scan rate (v) were plotted from these CVs. The diffusion coefficient of Li⁺ ions (D_Li) can be calculated from the linear relationship between I_p and v^1/2 according to the following equation.⁶⁵

I_p = 2.69 × 10⁵n^3/2AD_Li^1/2C^*_Liv^1/2 (4)

where n is the number of electrons per reaction species, it is 1 for Li⁺ in this reaction. A is the electrode area (cm²), the radius of electrode is 7 mm, so the area is 1.5368 cm², and C^*_Li is the bulk concentration of the Li⁺ ion in the electrode (mol cm⁻³), here the value is 7.4745 × 10⁻² mol cm⁻³. Fig. 12b, d and f show good linear relationships between I_p and v^1/2 for single SnO₂ microspheres, binary SnO₂/PPy, and ternary SnO₂/rGO/PPy anodes, respectively. Accordingly, the Li⁺ diffusion coefficient of the ternary anode is calculated to be 1.8 × 10⁻⁸ cm² s⁻¹ that is much higher than single SnO₂ (1.2 × 10⁻⁹ cm² s⁻¹) and binary SnO₂/PPy (7.4 × 10⁻⁹ cm² s⁻¹).²⁸ The improvement of diffusion coefficient of Li⁺ is due to the larger specific surface area and flexible structures of rGO in SnO₂/rGO/PPy nanocomposites, providing more intercalation/deintercalation Li⁺ active sites and facile Li⁺ transfer channels, promoting Li⁺ transfer and Li⁺ intercalation/deintercalation reaction. Meantime, rGO owns excellent electronic conductivity, the ternary SnO₂/rGO/PPy nanocomposites have rapid Li⁺ transfer and fast charge transfer simultaneously, so there is larger current in the electrode at the same scan rate. The calculated Li⁺ diffusion coefficient according to the equation is much higher than single SnO₂ and binary SnO₂/PPy. These results further confirm the excellent rate capabilities measured with the ternary SnO₂/rGO/PPy nanocomposites.

Fig. 12 Cyclic voltammetry of hollow SnO₂ microspheres (a), hollow SnO₂/21 wt% PPy core–shell nanocomposites (c) and hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites (e). The scan rates are 0.1, 0.2, 0.3, 0.4 and 0.5 mV s⁻¹, respectively. Plot of peak current (I_p) as a function of the square root of the scan rates (v^1/2) for hollow SnO₂ microspheres (b), hollow SnO₂/21 wt% PPy core–shell nanocomposites (d) and hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites (f).

To further identify the role of rGO and PPy in the composites, the morphology of standalone SnO₂ hollow spheres electrode and hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites electrode after cycling was studied by SEM, as shown in Fig. 13. Significant cracks were observed on the standalone SnO₂ hollow spheres electrode after 100 charge–discharge cycles, due to the large volume expansion and phase transition (Fig. 13a). Compared to the SnO₂ electrode, as shown in Fig. 13b, the hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposite electrode hardly has any crack after 100 cycles. These results indicate that the rGO and PPy buffer on the SnO₂ hollow spheres was able to relieve the large volume expansion and maintain the relative integrity of the electrode. Consequently, the cyclic stability of SnO₂/rGO/PPy composites electrode is greatly improved relative to SnO₂, due to the presence of rGO and PPy buffer, and effective matrix of graphene and synergistic effect of graphene and polypyrrole to yield a robust architecture for mitigating the accumulated strain in high-performance Li-ion batteries.

Fig. 13 SEM images of (a) a hollow SnO₂ spheres electrode after 100 cycles, and (b) a hollow SnO₂/21.5 wt% rGO/16.5 wt% PPy core–shell nanocomposites electrode after 100 cycles.

Conclusions

In this work, core–shell structured hollow SnO₂/rGO/PPy ternary nanocomposites were prepared via a hydrothermal method followed by an in situ chemical-polymerization of PPy. The ternary nanocomposites were studied as an anode material for lithium ion batteries, exhibiting significantly enhanced cycling performance (647.8 mA h g⁻¹ after 100 cycles) and rate capabilities (above 117.6 mA h g⁻¹ at current density of 3900 mA g⁻¹), compared to the standalone hollow SnO₂ microspheres and binary SnO₂/PPy anodes. The performance achieved is much higher than that reported for any other SnO₂ based anode material to date. The enhanced cycling performance is attributed to not only the hollow ternary core–shell structure and PPy buffer layers, but also the graphene framework enhancing the strength of the electrode. Compared to a SnO₂/PPy binary nanocomposite, Li⁺ diffusion coefficient in SnO₂/rGO/PPy ternary anode is improved by one order of magnitude due to the promotional role of rGO frameworks. Additionally, the large specific surface area and good conductivity of rGO also effectively enhance electron transfer in the nanocomposite, thus reducing electrochemical polarization and increasing reaction reversibility.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51402155), the Natural Science Foundation of Jiangsu (BK20141424) and the NUPT Scientific Foundation (NY215014, NY214183, NY214088, NY214021). G. W. gratefully acknowledge financial support from the New York State Center of Excellence in Materials Informatics and startup funds from the University at Buffalo, SUNY.

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