Hollow Sn–Ni nanoparticles coated with ion-conductive polyethylene oxide as anodes for lithium ion batteries with superior cycling stability

Abstract

A facile strategy is designed for the fabrication of hollow, Sn–Ni nanoparticles (NPs) surrounded by ion-conductive, polyethylene oxide (PEO) coating to address the structural and interfacial stability concerns facing Sn-based anodes. In the unique architecture of hollow Sn–Ni@PEO NPs, the ductile inactive Ni as a buffer matrix can alleviate the volume change of Sn. Moreover, the synergistic effect between the elastic ion-conductive PEO coating and the hollow interior forces the active Sn to expand inward into the hollow space in the lithiated state, and thus effectively accommodates the substantial volume expansion. In particular, the PEO coating not only suppresses the unfavorable aggregation and pulverization of Sn during cycling, but also helps in forming a stable solid electrolyte interface (SEI) film on the high surface area nanostructured electrodes. Benefiting from the structural features, hollow Sn–Ni@PEO NPs exhibit a reversible capacity of 584 mA h g⁻¹ after 100 cycles with excellent coulomb efficiency of higher than 99%, superior to the bare counterparts. The contribution to the excellent cycling performance by the PEO coating and the hollow structure is verified by galvanostatic charge/discharge cycling, cyclic voltammetry, electrochemical impedance spectroscopy and SEM measurements.

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1 Introduction

In the increasing demand for advanced lithium ion batteries,¹ Sn has been considered as a promising anode candidate due to its low cost, abundant nature and environmental friendliness, and more importantly, the high theoretical specific capacity (993 mA h g⁻¹).^2–4 However, the drastic volume change of Sn-based electrodes in the course of the battery operation would cause severe mechanical damage of electrodes and then lead to poor long-term cycling stability.^5,6 Up to now, various strategies have been developed to address these issues, such as optimizing structure or morphology.^7–9

Among various structures tailed to accommodate the volume change, hollow nanostructure is attracting fast growing interests due to its intriguing properties.^10–16 Lou et al.¹⁷ synthesized SnO₂ hollow nanospheres to prolong the cycle life of SnO₂ anode, showing excellent cycling performance. The enhancements in electrochemical performance are largely attributed to the well-defined interior void in hollow structure, which is capable of alleviating the structural strain and buffering against the volume expansion associated with repeated Li⁺ insertion/extraction process. The cavities in hollow nanostructured electrodes could provide extra space for the storage of Li⁺, beneficial for high loading capacity. In addition, the hollow architectured electrodes possess large surface area, low density and short effective path length for Li⁺ transport compared to solid counterparts, which can reduce the over potential and allow better reaction kinetics at the electrode surface.^18,19 Except for hollow structure, another effective strategy, used to overcome this issue, is to prepare Sn–TM intermetallic compounds (TM represents transition metal), in which introduction of an electrochemically inactive TM results in a suppression of the volume changes of the anode, caused by the active Sn.^20,21 Among these transition metals, Ni with favorable ductility is widely used in synthesizing Sn–Ni intermetallic compound anode for improving the cycling performance.²²

Even though mechanical fracture caused by volume expansion could be overcome by fabrication of Sn–Ni hollow structure to some extent, the stability of active Sn is still challenging, especially for anode subjecting to substantial volume change. One of the urgent issues associated with stability to be addressed is the aggregation and pulverization of Sn caused by the continual electrochemical cycling.²³ Fabrication of polymer coating on the surface of active materials is one of the effective methods to mitigate the problem.^24–26 The commonly coated polymer is classified into electron-conductive polymer and ion-conductive polymer.²⁷ As a unique class of coating, the ion-conductive polymer coating serves as a protective layer and provides the functionality in inhibiting the aggregation and disintegration upon extended cycling. Accompanying with buffer effect of ductile Ni element in Sn–Ni NPs and hollow interior, the intrinsic flexibility of the ion-conductive coating forces the active Sn to expand inward, minimizing the degradation of the mechanical integrity of the electrodes.²⁸ Another critical factor influencing the stability of electrodes is the SEI stability at the interface. Owing to the repetitive volume expansion and contraction of Sn, the SEI film formed in lithiated expanded state can be broken as the nanostructure shrinks upon delithiation. As a result, the passive SEI film will become thick with repeated breaking off and reforming upon extended cycling. The increase in SEI film thickness will lead to a large amount of Li⁺ irreversible trapped in the thick SEI film, leading to large irreversible capacity.^29–33 The coating of ion-conductive polymer, such as PEO, would be an effective surface modification method to smartly mitigate the problems mentioned above. Such polymer coating could not only avoid the direct contact between the active Sn and liquid electrolyte, but also prevent liquid electrolyte from decomposition to form SEI film.²⁸ Moreover, the PEO coating acting as separator/electrolyte functionality would be helpful to form a stable SEI film.³⁴ Nevertheless, the reports about coated-PEO on the hollow Sn–Ni intermetallic compound anode materials are limited.

Here, hollow Sn–Ni@PEO NPs are successfully synthesized via galvanic replacement reaction assisted with mechanical coating technique for the first time and evaluated as lithium ion battery anode. For hollow Sn–Ni@PEO NPs, the ductile Ni can cushion volume change caused by active Sn. The combination of PEO coating and Sn–Ni hollow structure provides double protection against the volume expansion of Sn. Moreover, the PEO as a protective electrolyte/separator layer endows the electrodes with structural integrity and cycling stability. When used as an anode material, hollow Sn–Ni@PEO NPs are expected to display extraordinary electrochemical performance with outstanding reversible capacity, high coulomb efficiency and excellent cycling stability, compared with their bare solid counterparts.

2 Experimental

2.1. Synthesis of Ni NPs

Ni NPs were synthesized by chemical reduction. 0.97 g nickel chloride (NiCl₂·6H₂O, >98%) was dissolved in 20 mL ethanol and the pH of the solution was adjusted to 12 using 2 M sodium hydroxide (NaOH, >96%) solution. 10 mL hydrazine monohydrate (N₂H₄, 80 wt%) used as reducing agent was added into the solution with vigorous stirring at 60 °C. Large bubbles arose immediately after adding the reducing agent, indicating the reduction reaction occurred. The mixed solution was stirred for 30 min. Then the Ni NPs were collected and washed with distilled water and ethanol for several times. The prepared Ni NPs were stored in ethanol for further synthesis.

2.2. Synthesis of hollow Sn–Ni NPs

0.50 g of as-prepared Ni NPs was initially dispersed in 150 mL distilled water and sonicated to achieve a homogeneous suspension. The suspension solution was immediately transferred into a 250 mL flask with vigorous stirring. Then 1.50 g stannous sulfate (SnSO₄, >99%) was added to the solution at 80 °C under argon gas protection and the mixed solution was stirred for 12 h. After cooling to room temperature, the precipitate was centrifuged and washed with distilled water and ethanol for several times. Following that, the products were annealed under argon atmosphere at 200 °C for 8 h. Finally, hollow Sn–Ni NPs were obtained.

2.3. Synthesis of hollow Sn–Ni@PEO NPs

0.10 g of the resultant hollow Sn–Ni NPs was redispersed in distilled water and 0.01 g sodium dodecyl sulfate (SDS) was added to the solution. The mixed solution was sonicated to achieve a homogeneous suspension. Then 0.05 g of PEO (M_w = 600 000) was dissolved in 20 mL water and the solution was dropped into the homogeneous suspension with strong magnetic stirring. After completely adding, the mixed solution was stirred for 3 h at room temperature. Finally, the products were collected and washed with ethanol for several times, followed by vacuum drying for 4 h. For comparison, solid Sn NPs without coating were also prepared.³⁵

2.4. Characterization

The morphology and microstructure of the obtained samples were examined by transmission electron microscopy (TEM, JEM100CXII, 5 kV) and field emission scanning electron microscopy (FESEM, S-4800) with energy dispersive X-ray spectrometer. The composition of samples was investigated using Genesis XM2 APEX 60SEM energy dispersive X-ray spectroscopy (EDS, USA). X-Ray powder diffraction (XRD, Rigaku D/MAX-2500) was used to examine the composite structure. It should be pointed that the hollow Sn–Ni NPs were washed with acetonitrile under sonicated conditions in glove box when used to prepare SEM sample, in order to clearly observe their morphology after 50 cycles. However, the Sn–Ni@PEO NPs were only sonicated in acetonitrile for a few seconds in case of the destruction of PEO layer.

Electrochemical measurements were carried out using two-electrode cells (CR2032) with lithium metal as the counter/reference electrode and the as-prepared samples as the working electrode. The working electrode was prepared by coating the mixture of active materials, acetylene black and polyvinylidene fluoride (8 : 1 : 1 by weight) in N-methyl pyrrolidinone onto Cu substrates, following by pressing at 10 MPa for 5 min and drying at 90 °C for 12 h. The electrolyte solution contains 1 M LiPF₆ in a mixture of ethylene carbonate and diethyl carbonate (1 : 1 by volume). Celgard 2400 was used as the separator. The cell assembly was performed in an Ar-filled glove box filled with moisture and oxygen concentrations below 1.0 ppm. Galvanostatic charge and discharge measurements were measured in the voltage range of 0.01–2.00 V by a LAND battery tester. Cyclic voltammetry (CV) was performed on a CHI 660B electrochemical workstation at a scan rate of 0.1 mV s⁻¹ in the potential window of 0.001–2.000 V. Electrochemical impedance spectroscopy (EIS) was measured with a frequency range of from 1 × 10⁶ to 0.01 Hz.

3 Results and discussion

The formation mechanism of hollow Sn–Ni@PEO NPs is schematically illustrated in Fig. 1. In the typical galvanic replacement reaction, the as-prepared Ni NPs are used as sacrificial templates and react with Sn²⁺ to prepare hollow Sn–Ni NPs. The driving force of the reaction comes from the difference in reduction potentials of Sn and Ni, with the potential of Sn²⁺/Sn (−0.136 V vs. SHE) is higher than that of Ni²⁺/Ni (−0.257 V vs. SHE). When the galvanic replacement is initiated, Ni atoms will be dissolved and Sn²⁺ will capture the electrons to generate Sn atoms on the surface. With the galvanic replacement reaction proceeding, hollow Sn–Ni NPs with interior void space are obtained.^36,37 In the facile coating process, SDS is introduced as surfactant and absorbed on the surface of hollow Sn–Ni NPs to generate a soft interface. With vigorous stirring, the PEO molecules gradually deposit on the surface of hollow Sn–Ni NPs and then generate a complete coating layer. It is anticipated that this method will be very suitable for to prepare other organic–inorganic anode materials.

Fig. 1 Schematic illustration of the formation process of hollow Sn–Ni@PEO NPs.

Fig. 2 shows the XRD patterns of the bare hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs. As shown in Fig. 2, all the diffraction peaks of bare hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs can be indexed to Ni₃Sn₄ phase (JCPDS no. 04-0845, space group: C2/m), Ni₃Sn₂ phase (JCPDS no. 07-0256, space group: P63/mmc) and Sn phase (JCPDS no. 04-0672). The peaks are sharp and intensive, implying the high crystallization of the samples. No residues or contaminants are detected.

Fig. 2 XRD patterns of (a) hollow Sn–Ni NPs and (b) hollow Sn–Ni@PEO NPs.

Fig. 3a and b show the respective microstructures of hollow Sn–Ni NPs before and after coating, respectively. The picture inset in Fig. 3a displays the microstructure of the as-prepared Ni NPs, which are sacrificial template of hollow Sn–Ni NPs. As shown in Fig. 3a, the bare hollow Sn–Ni NPs are spherical-shaped, which is originated from the morphology of Ni NPs. The diameters of hollow Sn–Ni NPs range from 240 nm to 250 nm, larger than that of Ni NPs (about 132 nm). This can be explained by the mechanism of galvanic replacement reaction, as shown in Fig. 1. Apparently, the hollow structure is clearly revealed by the contrast between the exterior shell and the interior void space, implying the hollow structure in Sn–Ni NPs has formed. Fig. 3b displays the typical microstructure of hollow Sn–Ni@PEO NPs after coating. The TEM image indicates that introducing PEO does not change the general spherical shape and the hollow architecture of hollow Sn–Ni NPs prototype, but results in increased surface toughness. As shown in Fig. 3b, the diameters of hollow Sn–Ni@PEO NPs are estimated to be about 278 nm on average, bigger than that of bare hollow Sn–Ni NPs. The increase in diameters further evidences the existence of PEO layer. Fig. 3c shows the EDS spectrum of hollow Sn–Ni NPs. From the EDS results, the Sn/Ni weight ratio of hollow Sn–Ni NPs is 22 : 3. According to the TG curve of hollow Sn–Ni@PEO NPs as shown in Fig. 3d, an estimable weight percentage of PEO is determined to be ca. 8%. Due to its electrochemical inactive nature, the content of PEO will be removed when calculate the specific capacity of the resultants.

Fig. 3 (a and b) TEM images of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs, respectively. (c) EDS spectrum of hollow Sn–Ni NPs. (d) TG curve of hollow Sn–Ni@PEO NPs.

Fig. 4 shows the CV curves of electrodes structured from hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs, respectively. As can be observed in Fig. 4a and b, the curves in both cases show the same characteristic current peaks, indicating the PEO coating does not bring changes to the composition of active materials. In both cases, there appear two reduction peaks at ca. 0.35 V and 0.65 V and two oxidation peaks at ca. 0.50 V and ca. 0.70 V associating with the alloying/dealloying process of the anode materials. In the first cycle of bare hollow Sn–Ni NPs as shown in Fig. 4a, there is a broad reduction peak centered at around 1.00 V. In particular, the broad initial reduction peak is irreversible and will result in large irreversible capacity in the first cycle. This should be closely ascribed to the electrolyte decomposition and the formation of SEI layer. As shown in Fig. 4b, the initial irreversible reduction peak of hollow Sn–Ni@PEO NPs is smaller than bare hollow Sn–Ni NPs, suggesting the lower initial irreversible capacity of hollow Sn–Ni@PEO NPs. Moreover, the subsequent cycles of hollow Sn–Ni@PEO NPs are almost overlapped, which implies that the alloying/dealloying process of hollow Sn–Ni@PEO NPs is highly reversible.³⁸

Fig. 4 Cyclic voltammetric curves of (a) hollow Sn–Ni NPs and (b) hollow Sn–Ni@PEO NPs.

Fig. 5a shows the initial charge/discharge behaviors of bare hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs at a constant current density of 100 mA g⁻¹ between 0.01 V and 2.00 V, respectively. The two different anode materials show potential profiles with similar characteristic, which is in agreement with the CV curves shown in Fig. 4. The two profiles both appear a long plateau around 0.35 V and the potential decreases gradually until 0.01 V, which is ascribed to the formation reaction of Li_xSn (x = 1–3).^39,40 Fig. 5b compares the cycling performance of hollow Sn–Ni@PEO NPs with bare hollow Sn–Ni NPs and solid Sn NPs. As expected, the electrochemical characteristic of hollow Sn–Ni@PEO NPs is significantly superior to that of bare counterparts. Hollow Sn–Ni NPs without coating deliver a discharge capacity of 419 mA h g⁻¹ after 100 cycles, whereas the cycling performance of solid Sn NPs is very poor. The initial coulomb efficiency of bare hollow Sn–Ni NPs is only 86% and the subsequent coulomb efficiency is no higher than 99%, as shown in Fig. 5c. On the contrary in the PEO-coated configuration, hollow Sn–Ni@PEO NPs exhibit a high initial discharge capacity of 920 mA h g⁻¹ with a high initial coulomb efficiency of 90% and maintain an excellent reversible capacity of 584 mA h g⁻¹ after 100 cycles with coulomb efficiency higher than 99%. This result suggests that there is a synergistic effect between hollow Sn–Ni NPs and PEO protective layer. The introduction of hollow structure along with the ductile Ni element can effectively accommodate the volume change of Sn, improving the electrochemical performance.²² The separation action of the PEO coating layer between electrolyte and hollow Sn–Ni NPs reduces the electrolyte decomposition, leading to a low consumption of lithium during lithiation/delithiation process, which results in a low capacity loss during every cycle.²⁸ Therefore, the hollow Sn–Ni@PEO NPs exhibit high initial coulomb efficiency and reversible capacity compared to bare hollow Sn–Ni NPs. Moreover, a facile pathway for Li⁺ transport can be provided by the ion-conductivity of PEO, which can effectively enhance the rate performance hollow Sn–Ni NPs during cycles.⁴¹ In order to evaluate this improved effect, the comparison of rate capability between hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs were displayed in Fig. 5d. As seen in the figure, the corresponding rate capability of two anode electrodes with the various rates stepwise increased from 0.1 C to 2 C and then switched back. In the first rate cycle of the Sn–Ni@PEO NPs electrode, the average reversible capacities are 1135, 750, 607, 471, and 364 mA h g⁻¹ at the increasing current densities of 0.1, 0.2, 0.5, 1, and 2 C, respectively. When the current densities decrease back from 2 C to 0.2 C, and 0.1 C, the reversible capacities recover from 364 mA h g⁻¹ to 628, and 707 mA h g⁻¹. Although the rate of 2 C was imposed on this electrode, its corresponding specific capacities were still as high as 364 mA h g⁻¹. The stability of Sn–Ni@PEO NPs is better than that of the Sn–Ni NPs which shows a dramatical drop at the high rate of 1 C, and the recovered reversible capacities at the rate of 0.2 C is only 200 mA h g⁻¹. This is because that the coating of ion-conductive polymer PEO not only can prevent the Sn–Ni NPs from the pulverization and collapse caused by large volume change, but avoid the direct contact between the active materials and liquid electrolyte, ensuring a facile Li⁺ transport, stable surface and thin SEI film.

Fig. 5 (a) Initial charge/discharge curves of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs. (b) Cycling performance of solid Sn NPs, hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs at 100 mA g⁻¹ in the voltage range from 0.01 V to 2.00 V. (c) Coulomb efficiency of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs at 100 mA g⁻¹ in the voltage range of 0.01–2.00 V. (d) Rate capability of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs at various current densities between 0.01 V and 2.00 V.

The excellent performance of hollow Sn–Ni@PEO NPs with good cycling stability could be attributed to the ductile Ni element, hollow structure and PEO coating. First, the ductile Ni element acts as a buffer matrix to relieve the stress arising from the volume change during the Li⁺ insertion process, and it also stabilizes the liberated Sn atoms after the Li⁺ extraction process. Second, the synergistic effect of PEO coating and hollow structure allows for volume change of Sn–Ni intermetallic compounds to expand inwards rather than outwards only, giving rise to the electrode integrity without mechanical breaking. Third, the exterior PEO coating prevents the direct exposure Sn with liquid electrolyte, and thus provides a static interface without detrimental surface effects. Furthermore, the PEO coating allows for the development of a stable SEI film on the high volume change Sn-based electrodes.^{26,34,42–44}

The validity of above speculation can be partly confirmed by EIS measurements. Fig. 6a and b show the EIS spectra of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs after 1st and 50th cycle at 100 mA g⁻¹, respectively. The typical characteristics of these Nyquist plots are observed with two semicircles in the high and middle frequency range and a sloping straight line in the low frequency. The first semicircle in high frequency range is classically assigned to the impedances from SEI film. As shown in Fig. 6a, the remarkable difference in diameters of the first semicircles for hollow Sn–Ni NPs after 1st cycle and 50th cycle is believed to have been due mostly to an increase in the SEI resistance, stemming from the thick SEI formation.⁴⁵ As shown in Fig. 6b, for hollow Sn–Ni@PEO NPs after 50th cycle, the diameter of the first semicircle is much smaller than that of hollow Sn–Ni NPs, implying the lower SEI resistance of hollow Sn–Ni@PEO NPs. The significant reduction in SEI resistance with cycling signifies an enhanced stable formation of SEI film, which is possibly achieved by the introduction of PEO layer, as mentioned above.

Fig. 6 The EIS spectra of (a) hollow Sn–Ni NPs and (b) hollow Sn–Ni@PEO NPs.

To further probe the contribution to superior cycling stability of hollow Sn–Ni@PEO NPs by PEO coating, SEM images of samples before and after cycling at 100 mA g⁻¹ are shown in Fig. 7. The picture inset in Fig. 7a displays the microstructure of Ni NPs. As shown in Fig. 7a and b, the pristine hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs show good morphology with high uniformity and good dispersibility. It is worth noting that a broken nanoparticle can be observed in Fig. 7b, which confirms that the layer of PEO is successfully coated on hollow Sn–Ni NPs. Fig. 7c and d show the microstructure of bare hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs after cycling. As shown in Fig. 7c, although the ductile Ni alleviates a part of mechanical stress induced by large volume change, the aggregation behavior of bare hollow Sn–Ni NPs can still be clearly observed. This can be attributed to lack of the protection of flexible PEO layer leading to outward volume expansion of Sn without exerting the action of hollow structure.⁴⁶ The outward volume expansion directly brings about aggregation of hollow Sn–Ni NPs during cycles, decreasing their cycling stability. On the contrary, the morphology of hollow Sn–Ni@PEO NPs after cycling shows less variation from the pristine samples and the agglomeration phenomenon is hardly observed, as shown in Fig. 7d. This is because that the flexible PEO coating layer restricts the outward volume expansion of hollow Sn–Ni NPs and forces them to expand inward into the hollow space to release stress in the lithiated state, avoiding structural destruction, alleviating agglomeration and improving cycling stability during cycles. Thus, the PEO layer plays key role in enhancing the cycling stability of hollow Sn–Ni NPs.

Fig. 7 (a and b) SEM images of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs before cycling, respectively. (c and d) SEM images of hollow Sn–Ni NPs and hollow Sn–Ni@PEO NPs after 50th cycle, respectively.

4 Conclusions

In conclusion, hollow Sn–Ni@PEO NPs comprising interior hollow Sn–Ni NPs and exterior ion-conductive PEO coating are successfully fabricated. Such a unique structure provides triple protection against the aggregation and volume change of Sn active materials, and ensures a static interphase between electrodes and liquid electrolyte. As a consequence, hollow Sn–Ni@PEO NPs are beneficial to suppress the aggregation and stabilize the SEI formation on the surface. As an anode material, hollow Sn–Ni@PEO NPs exhibit a reversible capacity of 584 mA h g⁻¹ with a coulomb efficiency over 99% after 100th cycle, which is superior to their counterparts. Such a protocol to construct PEO-coated hollow nanoparticles can be further extended to other metals or metal oxides for lithium ion batteries.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 51143009 and 51273145).

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