Three-dimensional porous nickel supported Sn–O–C composite thin film as anode material for lithium-ion batteries

Abstract

A three-dimensional porous nickel supported Sn–O–C composite thin film anode is fabricated by the galvanostatical electrodeposition of an active material, Sn, onto a patterned three-dimensional porous nickel current collector from an organic electrolyte. The unique structure enables the homogeneous coating of Sn–O–C composite thin film, 100 nm in thickness, on highly porous dendritic Ni particles. Along with the existence of inter-particle spacings, this could efficiently accommodate great volume changes caused by the lithiation and delithiation of active material, Sn. The morphology, crystalline structure and chemical composition of Sn–O–C composite are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy with an energy dispersive X-ray analyzer (TEM-EDX), respectively. The results demonstrate that the Sn–O–C composite is composed of agglomerated Sn nanocrystals, which are inhomogeneously distributed in the decomposition products of organic electrolyte. Moreover, the electrochemical behavior of the Sn–O–C composite anode is investigated by cyclic voltammetry (CV), galvanostatic charge/discharge tests and electrochemical impedance spectroscopy (EIS). This anode delivers a discharge capacity of 573.4 mA h (g of Sn)⁻¹, (0.0883 mA h cm⁻²) after 100 cycles at 0.2 C-rate. The augmentation in total SEI and interfacial charge transfer impedance adequately explained the gradual capacity fading as cycle number increases.

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Introduction

The great demand for high capacity, long cycle life lithium-ion batteries used in portable electronics and electric vehicles has prompted the research and development of non-carbonaceous anode materials.^1,2 Tin has been proposed to be one of the most promising anode materials due to its high theoretical capacity (Li_4.4Sn, 994 mA h g⁻¹),³ which is more than two times higher than that of commercial graphite anodes (LiC₆, 372 mA h g⁻¹). However, pure tin anodes suffer from a rapid capacity fading and serious pulverization issues caused by great volume change (∼300%) during charge (lithiation)/discharge (delithiation) processes.^4,5 In order to solve the problem, various attempts have been employed to accommodate the volume change of Sn, for the purpose of improving cycling stability.

To date, one strategy is to design substrates with micro/nano structure in order to enhance the adhesion strength between active material and current collector.^6–8 Another strategy is to fabricate Sn-based intermetallics to obtain improved electrochemical properties, such as Ni–Sn and Co–Sn electrodes.^9–12 The reason why Sn-based intermetallics could enhance electrochemical properties is that the other component acts as an inactive buffering matrix to efficiently accommodate volume change when Sn and Li form the Li_xSn alloy in the charge process.¹³ In addition, other attempts such as mesoporous structure,¹⁴ core–shell structure,¹⁵ nanostructured Sn-based anode materials^16–18 have been reported.

Recently, Osaka et al. have electrodeposited core–shell structured Sn–O–C composite anode material on a flat copper foil current collector from an organic electrolyte.^19–23 The electrodeposition technique enables a direct contact between the Sn active material and a current collector, and it exempts the use of conducting additives and polymer binders. The core–shell structured Sn–O–C composite anode prepared by electrodeposition from organic electrolyte containing LiClO₄ possesses a discharge capacity of 465 mA h (g of Sn)⁻¹ with 80% capacity retention after 100 cycles.²⁰ The discharge capacity only remains as 69 mA h (g of Sn)⁻¹ after 100 cycles when the Sn–O–C composite is electrodeposited from an organic electrolyte containing LiPF₆.²³ However, as the thickness of the film increases, the Sn–O–C composite film is easy to peel off from the flat copper current collector; nevertheless, the poor reproducibility of the core–shell structured Sn–O–C composite film also impedes its usage.

In this study, we report the fabrication of a three-dimensional porous nickel supported Sn–O–C composite thin film by a two-step galvanostatical electrodeposition technique from an organic electrolyte. The three-dimensional nano-metals (Ni or Cu) composed of numerous pores and dendritic walls have attracted tremendous attention because they can provide a strong mechanical support, high surface area, and short diffusion length for electrons and ions.^24–27 The highly porous structure with enhanced surface roughness could efficiently improve the adhesion forces between the active material and the substrate. Moreover, we also conducted experiments to optimize the electrodeposition parameters for the purpose of achieving an optimum performance. Fig. 1 shows the schematic of the typical fabrication procedure of a three-dimensional porous nickel supported Sn–O–C composite thin film. The design contains numerous size-fitted pores and dendrites with suited inter-spacing, which could function as a good buffer to accommodate the volume change of the active material during charge/discharge processes, as well as a high surface area with an enhanced roughness to improve the adhesion forces between the Sn–O–C composite and the current collector.

Fig. 1 Schematic of typical fabrication procedure of three-dimensional porous nickel supported Sn–O–C composite thin film.

Experimental

The preparation of a three-dimensional porous nickel supported Sn–O–C composite thin film anode involved a two-step galvanostatical electrodeposition. All the solvents and chemicals were of reagent quality and used without further purification. In the first step, a three-dimensional porous nickel was electrodeposited on a flat copper foil with a working area precisely controlled at 1.00 cm², and the other surface areas of the copper foil were covered with insulating tape. The entire electrodeposition process was conducted in an electrolytic bath containing 2 mol L⁻¹ NH₄Cl (Sinopharm Chemical Reagent, 99.5%) and 0.1 mol L⁻¹ NiCl₂·6H₂O (Sinopharm Chemical Reagent, 98.0%) at a pH value of 3.5 with a Pt counter electrode and Cu foil as a working electrode.^26,28 The Cu foil was immersed in 20 v/v% H₂SO₄ for 10 s, rinsed with deionized water, and dried with a heater blower before electrodeposition. The electrodeposition parameter was controlled at a current density of 3.0 A cm⁻² for 60 s at room temperature, and the electrolyte was violently stirred using a magnetic stirrer during the process. Then, the specimen was rinsed with deionized water, dried in vacuum chamber for 10 h and transferred to a glove box with an Ar atmosphere.

In the second step, the galvanostatical electrodeposition was conducted in a three-electrode electrolytic bath with 2.5 mmol L⁻¹ SnCl₂ (Alfa Aesar, 99%) dissolved in an ethylene carbonate (EC) : dimethyl carbonate (DMC) (1 : 1 v/v) electrolyte solution (Shenzhen Capchem Technology Co., Ltd, water content less than 20 ppm) containing 1 mol L⁻¹ LiPF₆. Both the preparation of organic electrolyte and electrodeposition process were conducted in the glove box with an Ar atmosphere and a dew point below −100 °C. The electrodeposition process was program controlled by an electrochemical workstation (CHI660E, Chenhua, Shanghai) with a Pt quasi-reference electrode, a Pt counter electrode and a three-dimensional porous nickel as a working electrode. A constant cathodic current of 0.025 mA cm⁻² was applied to pass a charge of 0.25 C cm⁻² for the galvanostatical electrodeposition of the Sn–O–C composite thin film. After the electrodeposition of the Sn–O–C composite thin film, the specimen was rinsed with dimethyl carbonate and dried in vacuum chamber overnight for further electrochemical measurements and characterizations.

The three-dimensional porous nickel supported Sn–O–C composite thin film electrode was transferred into another three-electrode glass cell containing 1 mol L⁻¹ LiPF₆ electrolyte dissolved in EC : DMC (1 : 1 v/v) (Shenzhen Capchem Technology Co., Ltd, water content less than 20 ppm). The galvanostatic charge/discharge tests were evaluated with a battery testing system (Kejing, Shenzhen) at room temperature using lithium metal foil as reference and counter electrodes, and the as-prepared Sn–O–C composite thin film electrode as the working electrode. The current density was controlled at 0.045 mA cm⁻² (0.2 C) in the potential range between 0.01 V and 1.50 V vs. Li/Li⁺. In order to highlight the promotion effect of the 3D highly porous dendritic structure on capacity retention, bare Sn–O–C composite on a simple flat Cu substrate is also synthesized and galvanostatically charged/discharged under the same experimental conditions with the 3D porous nickel supported Sn–O–C thin film anode. Furthermore, both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on the 3D porous nickel supported Sn–O–C thin film anode with an electrochemical workstation (CHI660E, Chenhua, Shanghai) at room temperature. CV measurement was conducted in a potential window of 0.01–1.50 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹. EIS was measured at an open circuit voltage upon reaching a full charge at 0.01 V vs. Li/Li⁺ with 10 mV voltage amplitude and a frequency range between 1 MHz and 0.01 Hz.

Field emission scanning electron microscopy (FESEM, Siron 200) equipped with an energy dispersive X-ray analyzer (EDX) was performed to analyze the surface morphology and chemical composition of the as-prepared Sn–O–C composite thin film. X-ray diffraction (XRD) was carried out by Ultima IV equipped with Cu Kα (λ = 0.1541 nm) radiation. The scanning range (2θ) was from 20° to 80° and scanning speed was 5° min⁻¹. The electrodeposited three-dimensional porous nickel supported Sn–O–C composite thin film was characterized by X-ray photoelectron spectroscopy (XPS, Axis Ultra Dld) in order to analyze the elemental composition and existing state. Finally, the crystallographic structure of the as-prepared Sn–O–C composite was characterized by transmission electron microscopy with an energy dispersive X-ray analyzer (TEM-EDX, JEM 2010).

Results and discussion

Fig. 2(a)–(c) show the SEM images of the electrodeposited three-dimensional porous nickel at different magnifications. A top view in Fig. 2(a) clearly shows that the porous nickel contains numerous homogeneously distributed pores and a network of walls. The inset graph in Fig. 2(a) shows the cross-sectional SEM image of the three-dimensional porous nickel, it demonstrates that the walls are composed of numerous dendrites and the thickness of porous nickel is approximately 20 μm. Fig. 2(b) shows that the average diameter of these pores is approximately 8 μm and the dendritic walls are composed of Ni particles with different inter-particle spacing. Fig. 2(c) shows the magnified SEM image of Ni particles with feature size ranging from several hundreds of nanometers to about one micrometer in diameter. It appears to suggest that the existence of the inter-particle spacing can efficiently accommodate serious volume changes in active materials during charge/discharge tests.

Fig. 2 Top view SEM images of the surface morphologies of the electrodeposited three-dimensional porous nickel (a) (inset image is cross-sectional SEM image), (b and c) and three-dimensional porous nickel supported Sn–O–C composite thin film (d) (inset image is cross-sectional SEM image), (e and f).

After the electrodeposition of Sn–O–C composite thin film, the specimen was rinsed with DMC and dried for SEM investigation. Fig. 2(d)–(f) show the SEM images of the three-dimensional porous nickel supported Sn–O–C composite thin film. Fig. 2(d) is the top view SEM image, the inset image is the cross-sectional SEM image, but no obvious change can be seen compared to Fig. 2(a). Fig. 2(e) shows that the average diameter of these pores remains approximately 8 μm without any obvious change compared with Fig. 2(b). However, compared with Fig. 2(c), a homogeneous thin film attached to the surface of highly porous dendritic Ni particles can be seen from Fig. 2(f), which is the electrodeposition result from the organic electrolyte.

The XRD patterns of the three-dimensional porous nickel supported Sn–O–C composite thin film are shown in Fig. 3(a). The peaks belonging to the tetragonal phase of tin are observed with 2θ values of 30.64°, 31.99°, 55.29°, 62.53°, 63.79°, 64.58°, 72.34°, 73.18° and 79.44°, which correspond to the 200, 101, 301, 112, 400, 321, 222, 411 and 420 crystal planes of β-tin (JCPDS # 04-0673), respectively. The other peaks that appeared in the XRD pattern belong to the three-dimensional porous Ni and Cu substrates. XPS measurement was also carried out to further analyze the composition and constitution of the as-prepared material. However, due to the fact that XPS analysis only reveals the elemental composition of the composite within several nanometers beneath the surface in order to avoid misconception caused by the surface oxidation of the material, the XPS measurement was conducted after etching for three minutes (the etching depth is about 8 nm) and the spectra are shown in Fig. 3(b) and (c). Peaks corresponding to C_1s, O_1s, Sn_3d5/2 and Sn_3d3/2 are observed. A peak located at the binding energy about 284.7 eV is observed in the C_1s spectrum, as shown in Fig. 3(b), which is assigned to C–C bond.²⁹ A peak at about 531.9 eV is observed in the O_1s spectrum, as shown in Fig. 3(c), which represents the oxygen is supposed to exist in the state of C–O–C. In Fig. 3(d), peaks located at 485.0 eV and 493.4 eV correspond to Sn_3d5/2 and Sn_3d3/2, respectively. The peak located at 485.0 eV is assigned to Sn–Sn bond. Considering the result of XRD that Sn exists in the tetragonal phase, which corresponds to Sn crystals, a conclusion can be safely drawn that the only crystalline phase is Sn metal. Furthermore, the decomposition of organic electrolyte during galvanostatical electrodeposition of the deposit results in the formation of C–C and C–O–C bonds. Therefore, XPS analysis reveals that the deposit is composed of Sn crystals, which are inhomogeneously distributed in the decomposition products of organic electrolyte.

Fig. 3 (a) XRD pattern of the as-prepared Sn–O–C composite thin film; (b–d) correspond to XPS spectra of C_1s, O_1s and Sn_3d5/2 after etching for 3 minutes, respectively.

The three-dimensional porous nickel supported Sn–O–C composite thin film is directly electrodeposited on the copper wire mesh for TEM investigation. Fig. 4(a) shows the elemental analysis of the deposit performed by EDX equipped in TEM. The result indicates that elemental Sn, O and C are detected. Then, the structure of the electrodeposited Sn–O–C composite is investigated using TEM image, as shown in Fig. 4(b). A continuous thin film attached to the surface of porous nickel with approximately 100 nm in thickness can be clearly observed, and agglomerated tiny Sn particles with irregular shapes and different sizes are inhomogeneously distributed in the Sn–O–C composite, as represented in the dark areas (not substrate). The inset image shows the selected area electron diffraction (SAED) pattern of the circled area highlighted with a white solid line in Fig. 4(b). The diffraction ring in the SAED pattern shows the characteristics of both amorphous and polycrystalline substances. The diffuse central spot is the diffraction result of amorphous decomposition products of organic electrolyte, and the diffuse diffraction rings indicate a polycrystalline Sn phase but with the characteristic size of Sn crystal at about several nanometers, herein we refer to it as Sn nanocrystal. Therefore, the electrodeposited Sn–O–C composite thin film is composed of agglomerated Sn nanocrystals that are inhomogeneously distributed in the amorphous decomposition products of the organic electrolyte, which can be further supported by the HRTEM analysis in the previous work.²⁰ The amorphous decomposition products could function as a good buffer to accommodate volume change in active material Sn during charge/discharge tests.

Fig. 4 (a) EDX analysis and (b) TEM image (inset image is SAED pattern of circle area in (b)) of the three-dimensional porous nickel supported Sn–O–C composite thin film.

The electrochemical cycling performance and coulombic efficiency of the three-dimensional porous nickel supported Sn–O–C composite thin film anode measured by galvanostatical charge/discharge tests at a current density of 0.045 mA cm⁻² (0.2 C) are shown in Fig. 5(a). The charge/discharge capacities are calculated based on the assumption that all the passed charges are used to reduce Sn²⁺ to Sn metal; thus, the net mass of Sn is 0.154 mg. The first cycle delivers a charge/discharge capacity of 3605.8 mA h (g of Sn)⁻¹ (0.5553 mA h cm⁻²) and 940.3 mA h (g of Sn)⁻¹ (0.1448 mA h cm⁻²). The extremely high charge capacity and low coulombic efficiency (26.1%) in the first cycle is due to the further reduction of oxidized state of active material Sn and solid electrolyte interphase (SEI) formation.^30,31 After the first cycle, the discharge capacity decreases rapidly until reaching a steady value about 590.9 mA h (g of Sn)⁻¹ (0.091 mA h cm⁻²) at the 8^th cycle, while the coulombic efficiency gradually increases to 79% at 8^th cycle. Afterwards, the discharge capacity remains relatively steady. Despite an infinitesimal decrease as cycle number increases, it remains as high as 573.4 mA h (g of Sn)⁻¹ (0.0883 mA h cm⁻²) at the 100^th cycle with a discharge capacity retention rate of 61%. On the other hand, the coulombic efficiency rises to 90% at the 18^th cycle and remains steady at about 93.5% in the following cycles with an uttermost value of 95.5%. The high discharge retention capacity of the Sn–O–C composite thin film anode is attributed to the highly porous dendritic structure of Ni with high surface roughness, which not only enhances the adhesion force between active material and current collector but also functions as a good buffer to accommodate volume change during charge/discharge tests.

Fig. 5 (a and b): Cycling performance of 3D porous Ni/Sn–O–C thin film anode and bare Sn–O–C composite on a simple flat Cu substrate with a current density of 0.045 mA cm⁻²; (c) charge/discharge potential profiles of 3D porous Ni/Sn–O–C anode between 0.01 V and 1.50 V vs. Li/Li⁺; (d) CV profiles of the anode in a potential window between 0.01 V and 1.50 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹ at the first four cycles.

In order to validate the fact that 3D highly porous dendritic structures can accommodate the volume change during the charge/discharge tests and improve the capacity retention, bare Sn–O–C on a simple flat Cu substrate is also electrochemically cycled at the same current density for comparison purposes. The charge and discharge capacity are both shown in Fig. 5(b). It is obvious that the bare Sn–O–C on a flat Cu substrate exhibits really a poor electrochemical performance. This anode delivers a passable charge capacity of 1054 mA h (g of Sn)⁻¹ (0.1624 mA h cm⁻²) and discharge capacity of 585 mA h (g of Sn)⁻¹ (0.0901 mA h cm⁻²) in the first cycle. The charge capacity drops dramatically in the following cycles and the discharge capacity increases a little in the subsequent two cycles, and then decreases rapidly after 4^th cycle. This anode possesses a discharge capacity lower than 100 mA h (g of Sn)⁻¹ after the 15^th cycle. Therefore, on one hand, bare Sn–O–C on a flat Cu substrate exhibits a really bad performance in contrast to the 3D porous Ni/Sn–O–C thin film anode, it is attributable to the great volume expansion and pulverization of active material, Sn, during electrochemical cycling. On the other hand, the sharp contrast indicates that three-dimensional highly porous structure can accommodate a great volume change of active materials during cycling and maintain a stable capacity.

Fig. 5(c) shows the 1^st, 2^nd, 10^th and 100^th potential profiles of the three-dimensional porous nickel supported Sn–O–C composite thin film anode during charge/discharge tests in the potential range between 0.01 V and 1.50 V vs. Li/Li⁺. In the 1^st charge process, the potential sharply drops to 1.85 V with the formation of a small pseudo plateau, and gradually drops to 1.7 V with a large plateau (the plateau at about 1.7 V disappears in the following cycles). Then, the potential rapidly drops to 0.4 V with a small plateau followed by a gradual decrease to 0.01 V. The plateau at 0.4 V can be clearly observed in the 2^nd charge process; however, a pseudo plateau at about 0.3 V and gradual decrease in potential towards 0.01 V are observed at 10^th and 100^th cycles. The obvious change in lithiation potential during charge process indicates the formation of a series of Li_xSn alloys and SEI film on freshly exposed interface created by mechanical cracking.²⁰ On the other hand, four pseudo plateaus at about 0.45 V, 0.6 V, 0.7 V and 0.8 V can be observed in the 1^st and 2^nd discharge processes. However, these plateaus cannot be easily distinguished and the potential profiles become considerably smoother after the 10^th discharge cycle. Thus, it is assumed that the reversible lithiation and delithiation reactions occur in the first several cycles and become irreversible after the 10^th cycle. This indicates that the active material Sn can be fully lithiated to the state of Li_4.4Sn and be fully delithiated to the original state of Sn in the first several cycles but can never be reverted to the state of Sn after the 10^th discharge cycle.

The CV profiles of the first four cycles recorded in a potential window of 0.01–1.50 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹ are shown in Fig. 5(d). During the 1^st charge process, a strong cathodic peak appears at about 1.5 V–1.7 V and it disappears after the second cycle, which is consistent with the charge/discharge potential profiles. This is highly possible due to the SEI film formed on the surface of Sn–O–C during electrodeposition. Furthermore, the two reduction peaks at about 0.7 V and 0.4 V are observed in the first four cycles in the cathodic branches, which correspond to the formation of Li_0.4Sn and Li_2.33Sn alloys, respectively. The gradual drop in potential from 0.4 V towards 0.01 V indicates the complete lithiation of active material to form Li_4.4Sn alloy.⁸ On the other hand, four oxidation peaks in the anodic branch at about 0.45 V, 0.6 V, 0.7 V and 0.8 V are observed, which indicate the successive formation of Li_2.33Sn, LiSn, Li_0.4Sn and Sn, respectively.^8,20 Furthermore, both the reduction and oxidation peaks have tendency to become considerably smoother and some of them would disappear in the subsequent cycles. In the end, based on the CV analysis of the first four charge/discharge cycles, the assumption that the reversible lithiation and delithiation reactions occur only at first several charge/discharge cycles can be verified.

Differential capacity curves for the 1^st, 10^th and 20^th cycles are shown in Fig. 6 to further certify the lithiation and delithiation potential during the charge and discharge processes. In Fig. 6(a), a large sharp peak at 0.4 V and a small peak at 0.7 V are observed in the 1^st charge process. However, the first peak shifts from 0.4 V to 0.3 V at the 10^th and 20^th cycles, and the second peak at 0.7 V disappears after the 20^th cycle. The variation tendency of lithiation potential at 0.3 V corresponds well to the potential profiles discussed above, and the gradual disappearance of lithiation potential at 0.7 V further certifies the assumption that reversible lithiation and delithiation reactions occur only at the first several charge/discharge cycles. Peaks at about 0.45 V, 0.6 V, 0.7 V and 0.8 V are observed in the differential capacity curves during the 1^st, 10^th and 20^th discharge processes, as shown in Fig. 6(b), but sharp peaks change to broad peaks as cycle number increases, i.e., at 0.7 V and 0.8 V for delithiation. The variation tendency in delithiation potential is consistent with the result of potential profiles, and it also indicates that the reversible multi-stage delithiation reactions are gradually replaced by irreversible reactions as cycle number increases.

Fig. 6 (a) Differential capacity curves of the as-prepared 3D porous nickel supported Sn–O–C composite thin film anode at 1^st, 10^th, 20^th cycles during (a) charge and (b) discharge processes.

Fig. 7 shows the impedances of the three-dimensional porous nickel supported Sn–O–C composite thin film anode after different cycles. The impedances are all measured upon reaching a full charge at 0.01 V vs. Li/Li⁺. All the impedance spectra have similar features: a medium-to-high frequency depressed semicircle and an inclined low frequency line, which corresponds well to the results of previously reported Sn anode.^32,33 The inclined line in the low frequency region represents the lithium diffusion impedance, and the depressed semicircle suggests an overlap between the SEI film and an interfacial charge transfer impedance. The results of impedance spectra measured after the 5^th, 8^th, 10^th and 50^th cycles reveal that the total SEI and interfacial charge transfer impedance augment as cycle number increases, which serve as a good interpretation of the gradual capacity fading during charge/discharge cycles.

Fig. 7 Nyquist plots of three-dimensional porous nickel supported Sn–O–C composite thin film anode measured at open circuit voltage upon reaching a full charge at 0.01 V vs. Li/Li⁺ at the 5^th, 8^th, 10^th, 50^th cycles.

In order to further understand the relationship between capacity fading and morphology change, the three-dimensional porous nickel supported Sn–O–C composite thin film anode is retained for SEM investigation at first completely lithiated state, first completely delithiated state and 100^th completely delithiated state. Fig. 8(a) shows the SEM image of Sn–O–C composite anode at first completely lithiated state; compared with Fig. 2(e), obvious volume expansion and dramatic decrease in Ni inter-particle spacing can be observed. The inset graph in Fig. 8(a) represents magnified SEM image of a Ni particle coated with Sn–O–C composite thin film at fully lithiated state, volume expansion is obvious and small cracks can be found on the surface area, which are consistent with the fully lithiated state of tin with over 300% volume expansion in the first cycle. Fig. 8(b) shows SEM image of Sn–O–C composite anode at first fully delithiated state; compared with Fig. 8(a), volume expansion phenomenon disappears and Ni inter-particle spacing recover to initial state but remain a little smaller than that illustrated in Fig. 2(e). The inset image in Fig. 8(b) represents magnified SEM image, high volume compression can be observed compared with inset image in Fig. 8(a); furthermore, many tiny cracks and voids appear on the surface area of Sn–O–C composite anode. Fig. 8(c) and (d) show the SEM images and magnified images of Sn–O–C composite anode at completely delithiated state after 100 cycles. Obvious shrinkage in both number and average diameter of pores can be observed in Fig. 8(c), and the pores seem to be filled to some extent. Despite the small cracks and voids on the surface, the structural integrity is well maintained without the pulverization of active materials even after 100 cycles. This accounts for the excellent electrochemical stability during cycling. On the other hand, the appearance of cracks and voids is supposed to be responsible for capacity fading during charge/discharge tests.

Fig. 8 Top view SEM images of three-dimensional porous nickel supported Sn–O–C composite thin film anode (a) at completely lithiated state and (b) at completely delithiated state in the first cycle (inset graphs are magnified SEM images); (c) low magnification and (d) high magnification image at completely delithiated state after 100 cycles.

Conclusions

In summary, a three-dimensional porous nickel supported Sn–O–C composite thin film anode is fabricated by a two-step electrodeposition method. The unique three-dimensional porous structure of Ni offers large surface area, strong mechanical support and high surface roughness, which enables the homogeneous electrodeposition of Sn–O–C composite material and reinforces the adhesion strength between active material and substrate. Therefore, the structural integrity of the specially designed electrode could be well maintained during the electrochemical cycling process. Furthermore, various characterization methods reveal that the Sn–O–C composite thin film anode is composed of agglomerated Sn nanocrystals that are inhomogeneously distributed in the decomposition products of organic electrolyte and the anode delivers a discharge capacity of 573.4 mA h (g of Sn)⁻¹ (0.0883 mA h cm⁻²) after 100 cycles at 0.2 C-rate. Thus, the unique substrate with patterned three-dimensional porous nickel, which can realize good adhesion for deposit, ion pass and conductivity, is a good technique for applying the Sn–O–C composite thin film to the anode of the lithium secondary battery. The electrodeposition method to fabricate anode material also provides a simple, low-cost and applicable method for mass production in the lithium-ion battery industry.

Acknowledgements

This work is sponsored by the National Natural Science Foundation of China (no. 21303100) and Shanghai Natural Science Foundation (no. 13ZR1420400).

Notes and references

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