A nanostructured SnO₂/Ni/CNT composite as an anode for Li ion batteries

Abstract

A SnO₂/Ni/CNT nanocomposite was synthesized using a simple one-step hydrothermal method followed by calcination. A structural study via XRD shows that the tetragonal rutile structure of SnO₂ is maintained. Further, X-ray photoelectron spectroscopy (XPS) and Raman studies confirm the existence of SnO₂ along with CNTs and Ni nanoparticles. The electrochemical performance was investigated via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge measurements. The nanocomposite has been used as an anode material for lithium-ion batteries. The SnO₂/Ni/CNT nanocomposite exhibited an initial discharge capacity of 5312 mA h g⁻¹ and a corresponding charge capacity of 2267 mA h g⁻¹ during the first cycle at 50 mA g⁻¹. Pristine SnO₂ showed a discharge/charge capacity of 1445/636 mA h g⁻¹ during the first cycle at 50 mA g⁻¹. This clearly shows the effects of the optimum concentrations of CNTs and Ni. Further, the nanocomposite (SnNiCn) shows a discharge capacity as high as 919 mA h g⁻¹ after 210 cycles at a current density of 400 mA g⁻¹ in a Li-ion battery set-up. Thus, the obtained capacity from the nanocomposite is much higher compared to pristine SnO₂. The higher capacity in the nanoheterostructure is due to the well-dispersed nanosized Ni-decorated stabilized SnO₂ along with the CNTs, avoiding pulverization as a result of the volumetric change of the nanoparticles being minimized. The material accommodates huge volume expansion and avoids the agglomeration of nanoparticles during the lithiation and delithiation processes. The Ni nanoparticles can successfully inhibit Sn coarsening during cycling, resulting in the enhancement of stability during reversible conversion reactions. They ultimately enhance the capacity, giving stability to the nanocomposite and improving performance. Additionally, the material exhibits a lower Warburg coefficient and higher Li ion diffusion coefficient, which in turn accelerate the interfacial charge transfer process; this is also responsible for the enhanced stable electrochemical performance. A detailed mechanism is expressed and elaborated on to provide a better understanding of the enhanced electrochemical performance.

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Introduction

The growing interest in electric vehicles has shifted research towards the development of lithium-ion batteries (LIB).¹ Preliminary areas of battery research include materials engineering for the preparation of cathodes, anodes, and electrolytes. Usually, graphite or lithium titanate (LTO) is used as an anode, lithium cobalt oxide (LCO) or lithium ferrous phosphate (LFP) is used as a cathode, and LiPF₆ is used as an electrolyte in commercial LIBs.² Nevertheless, to increase the efficiency and cyclability, and to reduce the charging time of batteries, these battery components (i.e., the anode, cathode, and electrolyte) are currently being tailored with respect to their material compositions via the exploration of substitute materials.³

The present era is dominated by semiconducting metal oxides, which have vast applications, e.g., in photocatalysts, gas sensors, optoelectronics, energy storage devices, etc.^4,5 Metal oxides such as SnO₂, Fe₂O₃, Co₃O₄, and MnO₂ are the most commonly used anode materials for LIBs.^6,7 Among these, SnO₂ nanomaterials show a comparatively higher theoretical specific capacity (1494 mA h g⁻¹).⁸ This theoretical capacity of SnO₂ is four times higher than that of current commercially used anode materials, e.g., graphite (372 mA h g⁻¹) and other potential metal oxide anodes.^9,10 The higher theoretical specific capacity of SnO₂ is due to an electrochemical reaction occurring in the cell in two steps: (i) a conversion reaction and (ii) an alloying reaction, as represented in eqn (1) and (2), respectively:^6,11

Sn₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O (1)

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

These two reactions contribute to the overall specific capacity of SnO₂, with individual specific capacities of 711 and 783 mA h g⁻¹, respectively. Although SnO₂ seems to be a promising anode material for use in LIBs, during charging/discharging it shows some drawbacks such as (i) poor electrical conductivity, (ii) volume expansion (∼350%), (iii) pulverization, (iv) the agglomeration of tin nanocrystals, and (v) low coulombic efficiency in the first cycle.^12,13 To overcome these drawbacks, researchers have tried to modify SnO₂ via synthesizing SnO₂ with a variety of morphologies, such as nanoparticles; hollow nanostructures; tube-like, rod-like, and wire-like morphologies; etc.^14,15 Efforts have also been made to prepare nanocomposites of SnO₂ with carbonaceous materials, transition metals, or other metal oxides as buffering matrices.^11,16 It is reported that the addition of a carbonaceous buffering matrix (e.g., carbon nanotubes (CNTs), graphene, carbon nanofibers (CNFs), etc.) can enhance the electronic conductivity of SnO₂ and the mechanical strength, also cushioning massive volume changes.^10,14,17,18 Similar observations were recorded in our earlier publication, which reported that upon the addition of carbon, stability in terms of the capacity is observed, where carbon accommodated the strain of volume change and increased the conductivity of pristine SnO₂.¹⁹ However, the mere addition of carbonaceous materials to SnO₂ does not overcome all the issues. As the addition of carbon only helps in addressing the volume expansion, the problem of the initial coulombic efficiency loss still persists. Therefore, to improve the reversibility of conversion reactions, there is a need for further modifications. The doping of SnO₂ with transition metals (TMs) is also found to be helpful due to their high conductivities.²⁰ The 3d transition metals (Cu, Ni, Co, and Mn) can enable highly reversible conversion reactions when added to SnO₂ nanomaterials.^16,21,22 The added TM can be useful for inducing oxygen vacancies in SnO₂ in the lithiated electrode, resulting in an increased coulombic efficiency of more than 83%. Previous results have demonstrated the advantage of incorporating transition metals into SnO₂ to overcome reversibility problems.¹¹

As per the literature, there are many reports on composites of SnO₂ with carbon and/or transition metals/oxides for battery applications. For example, Liang et al. demonstrated a hydrothermally synthesized Ni-doped SnO₂ nanosphere based anode material with an initial reversible capacity of 1267 mA h g⁻¹ at a charge–discharge rate of 0.2C and a stable reversible capacity of 674.8 mA h g⁻¹ after 35 cycles.¹¹ Kim et al. reported a Ni-added SnO₂–carbon nanofiber composite with capacity retention of 447.6 mA h g⁻¹ after 100 cycles.¹⁰ Zhou et al. reported SnO₂/CNT with a high reversible capacity of 596 mA h g⁻¹ after 200 cycles for enhanced lithium storage applications.⁸ Zhang et al. reported the synthesis of a three-dimensional graphene/carbon nanotube/SnO₂ hybrid architecture with an improved reversible capacity of 842 mA h g⁻¹ after 100 cycles at 0.2 A g⁻¹ for lithium storage.¹⁷ Tang et al. reported the synthesis of hierarchical CNF/SnO₂/Ni nanostructures as an anode material, showing a capacity of 542.8 mA h g⁻¹ at a current density of 200 mA g⁻¹.²³ Sun et al. have reported Sn–SnO₂ hybrid nanoclusters embedded in carbon nanotubes with enhanced electrochemical performance, i.e., 1059 mA h g⁻¹ at 1 A g⁻¹ and 960 mA h g⁻¹ at 5 A g⁻¹ over 1000 cycles.²⁴ Cui et al. reported Sb-doped SnO₂/graphene–CNT based high-performance Li-ion and Na-ion battery anodes delivering rate capabilities of 659 mA h g⁻¹ at 10 A g⁻¹ with capacity retention of 73% after 1000 cycles at 1 A g⁻¹ in the case of LIBs, and 74% of the capacity was retained after 500 cycles in the case of SIBs.²⁵ Ma et al. studied a composite of carbon-nanotube-connected SnO₂/reduced graphene clusters as a highly reversible anode material for lithium-/sodium-ion batteries; at a current density of 0.1 A g⁻¹, SnO₂/rGO/CNT shows initial discharge and charge capacities of 1678.6 and 1091.2 mA h g⁻¹, respectively.²⁶ Wang et al. designed a hybrid nanostructure of SnO₂/Co₃O₄/rGO to act as an anode material for lithium ion batteries, exhibiting a capacity of 641 mA h g⁻¹ at a high current density of 1000 mA g⁻¹ after 900 cycles.²⁷

In this report, we have effectively fabricated a chemically engineered SnO₂/Ni/CNT anode using a simple hydrothermal method, and its performance was compared with pristine SnO₂. The preparation method for the SnO₂/Ni/CNT composite based anode material was also optimized in terms of the percentages of CNT and Ni in the composition with respect to the specific capacity. As per our knowledge and based on a literature survey, our nanocomposite anode electrode exhibits an impressive initial reversible capacity of 2329 mA h g⁻¹ with a specific capacity of 1180 mA h g⁻¹ even after 210 cycles at 400 mA g⁻¹.

Experimental methods

Material preparation

All reagents were analytical-grade reagents and were used without further purification. In a typical synthesis method, 4 mmol of stannic chloride (SnCl₄·5H₂O, 99%, Fisher Scientific) was dissolved 20 mL of distilled water, and 28 mmol of sodium hydroxide (NaOH, 99% S.D. Fine) dissolved in 20 mL of distilled water was added, and the mixture was stirred for 20 min. The resulting white solution was then transferred to an 80 mL-capacity Teflon autoclave and kept at 180 °C for 12 h. This pristine SnO₂ sample is named SnO. For the SnO₂/Ni nanocomposite, the addition of nickel nitrate (Ni(NO₃)₃) is carried out. 4 wt% Ni was obtained via dissolving a suitable amount of nickel nitrate in a minimum amount of distilled water, and addition was done in situ to the SnO reaction solution. This sample is named SnNi. For SnO₂/CNT, the addition of 1 wt% commercial CNTs (the activation of the CNTs was carried out before use) was done in situ to the SnO reaction solution. This sample is named SnCn. For the preparation of SnO₂/Ni/CNT, 4 wt% Ni and 1 wt% activated CNTs were added in situ to the SnO reaction solution. This sample is named SnNiCn. Also, for comparison, we synthesized and analyzed samples of SnNi, SnCn, and SnNiCn with different wt% levels in their compositions, which are named SnNi-2% (2 wt% Ni), SnNi-6% (6 wt% Ni), SnCn-0.5% (0.5 wt% CNTs), SnCn-1% (1 wt% CNTs), SnNiCn-2% (2 wt% Ni and 1 wt% CNTs), and SnNiCn-6% (6 wt% Ni and 1 wt% CNTs). The as-prepared precipitate was separated via centrifugation, washed with distilled water several times, and finally washed with ethanol. The product was dried at 80 °C overnight. The dried products of SnNi, SnCn, and SnNiCn were annealed at 550 °C for 3 h under a nitrogen atmosphere at a rate of 5 °C min⁻¹. Due to annealing, the Ni oxides get reduced to Ni, which is formed during hydrothermal treatment, and the carbonization of the CNTs occurs.⁸ These SnO, SnNi, SnCn, and SnNiCn samples are used for further analysis and comparisons hereafter.

Structural and morphological characterization

The crystal structures of all the synthesized anode nanostructures were examined via powder X-ray diffraction techniques (XRD, Bruker Advanced D8) using a Cu Kα radiation source. Morphological and micro-structural analysis of all as-synthesized nanostructures was carried out via field-emission scanning electron microscopy (FESEM, Hitachi, S-4800) and field-emission transmission electron microscopy (FETEM, JEOL JEM-2200FS). The surface chemical compositions were studied via X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co., Theta Probe). Room-temperature micro-Raman scattering analysis was performed using a HR 800-Raman spectrometer (Horiba Jobin Yvon), with excitation at 532 nm. Particle sizes were calculated using Particle Sizing Systems apparatus (Santa Barbara, Calif., USA).

Electrochemical measurements

To perform electrochemical measurements, 2032-type coin cells were used. The active material, conducting carbon, and polyvinylidene fluoride (PVDF) binder were mixed at a weight ratio of 80 : 10 : 10 with N-methyl-2-pyrrolidone (NMP) as a solvent. The prepared slurry was cast onto a copper current collector with the help of a doctor blade and was dried in a vacuum oven for 12 h at 120 °C before being used as a working electrode. After drying under ambient conditions, round discs 16 mm in diameter were punched out and further dried under vacuum at 120 °C. The 2032-type coin half-cells were assembled in an argon-filled glove box, using metallic lithium foil (75 μm in thickness) as the counter and reference electrode, quartz filter paper as the separator, 1 M LiPF₆ in ethylene carbonate (EC) : dimethyl carbonate (DMC) (1 : 1 in volume) (BASF) as the electrolyte, and the round discs as the working electrodes. Electrochemical measurements (galvanostatic charge–discharge tests) were carried out using a battery analyzer (MTI) (vs. Li/Li⁺) at room temperature. The cyclic voltammetry behaviour of the half cells was tested using a potentiostat/galvanostatic instrument (Metrohm Autolab) between 0.005 V and 3 V with a scan rate of 0.2 mV s⁻¹. Electrochemical impedance spectroscopy measurements were performed at an amplitude of 5 mV with a frequency range of 0.1 Hz to 1 MHz.

Results and discussion

Fig. 1a displays the X-ray diffraction (XRD) patterns of pristine SnO₂, SnO₂/Ni, SnO₂/CNT, SnO₂/Ni/CNT nanocomposite samples. All samples show strong and sharp peaks at 2θ values of 26.7, 34.1, 37.96, 51.7, 54.6, 58.9, 62.23, and 65.5°, which correspond to the (110), (101), (200), (211), (220), (310), (112), and (301) lattice planes of SnO₂ (JCPDS no. 072-1147) with a tetragonal rutile structure, and metallic Ni peaks (JCPDS no. 004-0850) are not obvious. No diffraction peaks from crystalline by-products, such as NiO, were detected. No other peaks are observed, indicating that Sn(OH)₄ formed first, and the further dehydration of Sn(OH)₄ generated SnO₂ nanoparticles.

Fig. 1 (a) X-ray diffraction patterns and (b) Raman spectra of SnO, SnNi, SnCn, and SnNiCn samples.

After calcination at 550 °C for 3 h and upon the introduction of Ni and CNTs, the characteristic peaks become wider and shift toward lower 2θ values in the case of SnO₂/Ni/CNT. This could be attributed to a decrease in the grain size and the insertion of smaller radius atoms, like Ni²⁺. Moreover, the peak shifts indicate that the as-prepared product is not a physical mixture but that the Ni species are incorporated into the lattice of SnO₂. However, the only observed diffraction peaks belong to SnO₂, as shown in Fig. 1a. This further confirms the formation of a single phase resulting from the successful doping of Ni into SnO₂. The slight shifts toward lower 2θ values are believed to be related to a decrease in the length of the SnO₂ crystal lattice, which is attributed to the lower ionic radius of Ni²⁺ (0.69 Å) compared with Sn⁴⁺ (0.83 Å). This further confirms that the Ni ions are incorporated into SnO₂ (ref. 16) (XRD patterns of SnNi-2%, SnNi-6%, SnCn-0.5%, SnCn-1.5%, SnNiCn-2%, and SnNiCn-6% are given in the ESI (Fig. S1†)). To confirm the presence of Ni and CNTs in the nanocomposite samples, we carried out further analysis.

All samples were also characterized based on Raman spectra, obtained at an excitation laser wavelength of 532 nm, as illustrated in Fig. 1b. For the undoped SnO₂ sample (pristine SnO₂), typical modes positioned at A_1g = 626 cm⁻¹ and B_2g = 774 cm⁻¹ were detected, which indicate the expansion and contraction of the Sn–O bond.²¹ In the case of Ni-doped SnO₂, the A_1g peak shifted to 635 cm⁻¹ and the B_2g peak shifted to 884 cm⁻¹ with a decrease in the peak intensity due to Ni²⁺ doping. The above phenomena provide powerful evidence of the incorporation of nickel into the SnO₂ cassiterite lattice. Moreover, no Ni–O characteristic peak (at about 570 cm⁻¹) was detected in the spectra of the Ni-doped samples.²² SnCn and SnNiCn showed characteristic D and G bands at around 1332 and 1581 cm⁻¹, and 1322 and 1591 cm⁻¹, respectively, which represent the carbon backbone. The peaks at around 575 cm⁻¹ are attributed to the Raman modes of the tin oxide nanoparticles in SnCn, while the peaks at 555, 635, and 864 cm⁻¹ in the case of SnNiCn, with shifts in the A_1g and B_2g mode peaks, are due to the doping of Ni²⁺ into the SnNiCn sample.²⁸ The peak at 2686 cm⁻¹ in SnCn was ascribed to the C–H bonds and long-range graphitic order, arising due to a second-order scattering process (a G′ or 2D band).²⁹

Fig. 2 shows the morphologies of the as-synthesized samples. Fig. 2a–h highlights FESEM images of all synthesized samples, i.e., pristine SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT, respectively. The morphology of pristine SnO₂ (Fig. 2a and b) takes the form of bricks with a width of 60 nm, which are formed due to the stacking of nanoplates.

Fig. 2 FESEM images of all the samples: (a and b) SnO; (c and d) SnNi; (e and f) SnCn; and (g and h) SnNiCn.

Fig. 2c and d shows the SnO₂/Ni nanocomposite, which shows nanorods (20–25 nm in width and 50 nm in length) of SnO₂ decorated with Ni nanoparticles. However, the size of the Ni nanoparticles is too small to determine because of the limitations of FESEM. There was no change in the morphology and size upon increasing the Ni concentration (for SnNi-2 and 4%, see the ESI (Fig. S2†)). This indicates that there is control of the growth of SnO₂ due to Ni and, hence, slightly different morphology is observed (from plates to nanorods). SnO₂/CNT (Fig. 2e and f) shows clusters of nanoplates (50–80 nm) of SnO₂. These nanoplate edges are seen vertically, and they look like rods. Further, SnO₂/Ni/CNT (Fig. 2g and h) shows nanoplates of SnO₂ embedded with CNTs and with Ni nanoparticles decorated on the same (for SnNi-2%, SnNi-6%, SnCn-0.5%, SnCn-1.5%, SnNiCn-2%, and SnNiCn-6%, see the ESI (Fig. S4†)). There is not much difference in the morphologies of samples with different percentages of Ni and CNTs.

Fig. 3 shows FETEM images of all the synthesized samples. Fig. 3a and b shows FETEM images of pristine SnO₂, showing the elongated nanoplates of SnO₂ (length: 150 nm). The inset of Fig. 3a shows the SAED pattern, which clearly reveals the crystalline tetragonal structure of SnO₂. Fig. 3c shows the EDAX spectrum of SnO₂, revealing the presence of Sn and O. Fig. 3d and e shows FETEM images of the SnO₂/Ni nanocomposite, showing SnO₂ nanorods having a length of 50 nm and a diameter of 10 nm, with Ni particles that are 5–7 nm in size. The inset of Fig. 3d shows the SAED pattern, which clearly reveals the crystalline tetragonal structure of SnO₂ with Ni, which matches with the standard JCPDS data. Fig. 3f shows the EDAX spectrum of SnO₂/Ni, revealing the presence of Sn, O, and Ni. Fig. 3g and h shows FETEM images of the SnO₂/CNT nanocomposite, which shows a combination of nanotubes with a diameter of 10 nm and a length of 80–100 nm, and 100 nm plate-like SnO₂. The inset of Fig. 3g shows the SAED pattern of SnO₂/CNT. Fig. 3i shows the EDAX spectrum of SnO₂/CNT, revealing the presence of Sn, O, and C. Fig. 3j and k shows FETEM images of the SnO₂/CNT/Ni nanocomposite, with CNT tubes and SnO₂ nanoplates (50 nm), and with tiny Ni particles (5 nm) deposited on the material. The corresponding SAED pattern is given in the inset of Fig. 3j, showing tetragonal SnO₂. In addition, the EDAX spectrum (Fig. 3i) shows the presence of Sn, O, Ni, and C in the same sample. Moreover, the selected-area electron diffraction (SAED) patterns of the Ni-doped samples, i.e., SnNi and SnNiCn (Fig. 3d and k), show characteristic diffraction rings, which were assigned to the (101) and (200) crystal planes of tetragonal SnO₂. This data is in good agreement with the XRD results.

Fig. 3 FETEM images of all the samples: (a and b) SnO; (d and e) SnNi; (g and h) SnCn; and (j and k) SnNiCn. EDAX spectra of all the samples: (c) SnO; (f) SnNi; (i) SnCn; and (l) SnNiCn.

Fig. 4a–d shows the interplanar crystal spacing of tetragonal rutile SnO₂ crystals (0.334 nm for the (101) plane) and Ni. The measured Interplanar distance shows the existence of tetragonal rutile SnO₂ along with Ni (0.203 nm for the (111) plane).

Fig. 4 High-magnification FETEM images of (a) SnO, (b) SnNi, (c) SnCn, and (d) SnNiCn.

The elemental mapping of SnNi, SnCn, and SnNiC (Fig. 5) shows the coexistence and uniform distribution of Sn, O, C, and Ni in the respective samples.

Fig. 5 Elemental mapping of all samples: Sn, O, and Ni for the SnNi nanocomposite; Sn, O, and C for the SnCn nanocomposite; and Sn, O, Ni, and C for the SnNiC nanocomposite.

X-ray photoelectron spectroscopy (XPS) analysis of the prepared SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT samples was carried out to study the oxidation chemical states of the elements. The fitted peaks in the XPS spectra were separated using XPS Peak 4.1 software.

Fig. 6a shows the survey spectra of SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT. The carbon 1s spectrum at 284.8 eV was used as a reference for calibration. The elemental compositions and oxidation states of the nanomaterials were investigated via XPS. Fig. 6b and 7 display spectra of SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT, confirming the presence of Sn, O, C, and Ni elements. The C 1s peak intensities for SnCn and SnNiCn were strengthened because of the introduction of CNTs. Fig. 6b shows the Sn 3d orbitals of SnO₂, SnO₂/Ni, SnO₂/CNT, and the SnO₂/Ni/CNT nanomaterial. Fig. 6b confirms the existence of Sn 3d_5/2 and Sn 3d_3/2 peaks for all four materials with binding energies of 486.2 and 494.6 eV; 486.8 and 495.2 eV; 486.5 and 494.9 eV; and 487.6 and 496 eV for SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT nanomaterials, respectively. For SnNi and SnNiC, however, these peaks are shifted to higher binding energies because of the Ni ions in SnO₂.

Fig. 6 XPS analysis: (a) survey spectra and (b) the Sn regions of SnO, SnNi, SnCn, and SnNiCn.

Fig. 7 XPS analysis: the O 1s regions for SnO, SnNi, SnCn, and SnNiCn; the C 1s regions for SnCn and SnNiCn; and the Ni 2p regions for SnNi and SnNiCn.

Fig. 7 shows the O 1s, C 1s and Ni 2p regions for SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT samples. The high-resolution O 1s spectra of all four nanomaterials can be deconvoluted into three peaks with binding energies of 530.2, 530.7, and 531.7 eV for SnO; 529.3, 530.4, and 531.6 eV for SnNi; 531.3, 531.8, and 532.6 eV for SnCn; and 529.4, 531, and 533 eV for SnNiCn.²¹ The C 1s spectra of the prepared SnCn and SnNiCn samples could be deconvoluted into different peaks, i.e., 284.7, 285.1, and 286.4 eV; and 282.6, 283.7, 284.9, and 286.2 eV, respectively. The strong C 1s peaks at 284.7 and 283.7 eV correspond to C–C bonds in the carbon nanotubes. The peaks located at 285.1 and 286.4 eV, and at 284.9 and 286.2 eV are related to carbon in epoxide C–O groups and carboxyl C=O carbon, respectively.⁹ The Ni 2p core-level spectra of SnNi and SnNiCn show doublet components, which are demonstrated in Fig. 7. The peak located at 855.4 eV for SnNi and those at 854 and 851.7 eV for SnNiCn are close to the binding energies of Ni 2p_3/2 and Ni 2p_1/2, which indicates the presence of zero valent Ni.³⁰ The other peak of SnNi at a binding energy of 861.7 eV is ascribed to a satellite peak.

The possible mechanism of formation of the SnO₂/Ni/CNT nanocomposite is discussed below. Pristine SnO₂ is formed as per the following reaction (eqn (3)), where NaCl is formed as a by-product.

SnCl₄ + 4NaOH → SnO₂ + 4NaCl + nH₂O (3)

SnCl₄ + 4NaOH + Ni(NO₃)₃ + CNT → Ni@SnO₂@CNT + 4NaCl + 3HNO₃ + nH₂O (4)

In the formation of SnO₂/Ni/CNT using tin chloride, initially SnO₂ is formed via the decomposition of tin hydroxide, which is favourable. Later, Ni hydroxide is formed along with SnO₂. During calcination at 550 °C under an inert atmosphere, Ni hydroxide is decomposed into Ni nanoparticles. Since the carbon nanotubes are inert, they do not react with any precursors, and SnO₂ and Ni are deposited onto the CNTs (eqn (4)).^22,23

Electrochemical studies

Furthermore, to understand the electrochemical process, cycle voltammogram (CV) curves of all four electrodes (SnO, SnNi, SnCn, and SnNiCn) for the first three cycles were analyzed in the voltage range of 0.005–3 V vs. Li/Li⁺ at a scan rate of 0.2 mV s⁻¹. The CV behaviour is in accordance with that reported previously, implying a similar lithiation/delithiation process.¹³ Fig. 8 shows that the CV curves of SnNi (Fig. 8b), SnCn (Fig. 8c), and SnNiCn (Fig. 8d) are in good agreement with the characteristics of pristine SnO₂ (Fig. 8a). This indicates that the modification of SnO₂ with CNTs and Ni did not change the electrochemical reaction process. The strong irreversible reduction peaks at around 0.68, 0.84, 0.82, and 0.81 V for SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT, respectively, during the first cycle, were due to their reversible reduction of SnO₂ to Sn, as given in the following equation:

Sn_1−x + 4Li⁺ + 4e⁻ → Sn_1−xNi_x + 2Li₂O (5)

Fig. 8 The first three consecutive CV curves of SnO, SnNi, SnCn, and SnNiCn at a scan rate of 0.2 mV s⁻¹.

In subsequent cycles, this peak no longer appeared, and irreversible capacity loss during the initial cycles was mostly due to the formation of Li₂O or solid electrolyte interface (SEI) layers. In the first cycle, cathodic peaks were detected at about 0.145, 0.15, 0.15, and 0.16 V for SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT, respectively, and an anodic peak at 0.55 V was detected, which corresponded to reversible alloying and dealloying, as described in eqn (5).

Sn_1−xNi_x + yLi⁺ + ye⁻ → Li_ySn_1−xNi_x (0 ≤ y ≤ 4.4) (6)

Oxidation peaks observed at around 1.22, 1.27, 1.25, and 1.27 V and reduction peaks to form the Li_ySn_1−xNi_x alloy at around 1.03, 0.85, 1.0, and 1.07 V for SnO₂, SnO₂/Ni, SnO₂/CNT, and SnO₂/Ni/CNT, respectively, were assigned to the partial reversibility shown in eqn (6). This implied that the overall Li storage capacity of the SnO₂ anodes increased. From the second cycle onwards, the CV curves almost overlap, suggesting the excellent reversibility of the electrochemical reactions. The addition of CNTs and doping with Ni prevented SnO₂ from cracking, leading to an improvement in the redox behaviour.^8,17,22

Electrochemical impedance spectroscopy (EIS) analysis was conducted to study the conductivity of ion transportation at the interface between the electrode and electrolyte. Fig. 9 shows AC impedance spectra of all SnO₂-based electrodes, which were measured under identical conditions. Fig. 9a shows the Nyquist plots of all samples. These plots display semicircles in the high-frequency regions, with inclined lines in the low frequency regions. The EIS spectra show that the charge transfer resistance (R_ct) of SnNiCn is only 7.46671 Ω, lower than the values for pristine SnO₂ (125.8174 Ω), SnO₂/Ni (102.255 Ω) and SnO₂/CNT (74.10349 Ω), indicating the better electronic conductivity of SnNiCn.

Fig. 9 (a) Electrochemical impedance spectra and (b) the relationship between Z_re and ω^−1/2 from charge discharge data for all samples.

The corresponding lithium diffusion coefficients are calculated on the basis of eqn (S4).†^19,31,32 It can be clearly seen that the diffusion coefficients of the SnNiCn electrode vary within the range of 10⁻¹³ to 10⁻¹¹ cm² s⁻¹, higher than the average value for pure SnO₂. SnO₂ with CNTs and Ni facilitates the penetration of the electrolyte and shortens the diffusion paths of lithium ions, thereby achieving good diffusion kinetics.

Fig. 10 shows the charge–discharge curves of all the optimized nanomaterials, i.e., pristine SnO₂ (SnO), SnO₂/Ni (SnNi), SNO₂/CNT (SnCn), and SnO₂/Ni/CNT (SnNiCn), at a current density of 50 mA g⁻¹ in a voltage window of 0.005–3 V (vs. Li/Li⁺). The theoretical specific capacity of the composite was calculated to be 491.5 mA h g⁻¹ based on the reversible capacities of the CNTs (372 mA h g⁻¹) and SnO₂ (1494 mA h g⁻¹). SnO, SnNi, SnCn, and SnNiCn exhibited first charge capacities of 636, 836, 1486, and 2267 mA h g⁻¹, respectively. The corresponding discharge capacities are observed to be 1445, 1564, 3098, and 5312 mA h g⁻¹, respectively, for the first cycle, with initial coulombic efficiencies of 44, 53, 48, and 42%, respectively. Two poorly defined plateaus (at 0.7 and 1.25 V) can be identified in the charge–discharge profiles, which are in good agreement with the CV results. The degradation in the initial capacity indicates Li-ion irreversibility, which is ascribed to the formation of a solid electrolyte interface (SEI) layer and is also due to possible side reactions between lithium ions and residual functional groups of the carbon nanotubes. The SEI layer, which is generated on the carbon nanotubes and SnO₂ in the form of Li₂O, is an electronic insulator and hinders electron transfer. SnO₂/Ni/CNT shows first discharge and charge capacities of 5312 mA h g⁻¹ and 2267 mA h g⁻¹ in the first cycle at 50 mA g⁻¹. The coulombic efficiency (CE) of the first cycle is 43%, and the large loss of capacity results from the formation of the SEI film.¹² The reported initial capacity is more than calculated theoretically because the theoretical capacity corresponds exclusively to the conversion reaction (711 mA h g⁻¹) or the alloying reaction (783 mA h g⁻¹). It is indicated that the initial discharge capacity, which is much higher than the theoretical value, when LiO₂-based reactions are entirely irreversible, is almost entirely retained in the second cycle. This large capacity might be due to the increasing reversibility of Li⁺ insertion and extraction, particularly the reversibility of Li⁺ extraction from Li₂O, and to the formation of the SEI film.⁷ Cyclic voltammetry showed that SEI formation occurred only in the first cycle, as indicated by the reduction peak near 0.9 V (vs. Li/Li⁺) during the first cycle. The further decrease in capacity is attributed to the pulverization of SnO₂ caused by the drastic volumetric change of SnO₂ during the insertion and extraction of Li ions. Well-dispersed nanosized-Ni-decorated stabilized SnO₂ in combination with CNTs avoids pulverization due to the minimized volumetric changes of the nanoparticles. It shows the accommodation of huge volume expansion and avoids the agglomeration of nanoparticles.¹⁰

Fig. 10 Electrochemical properties analysis. The initial discharge–charge profiles of electrodes of all samples: SnO, SnNi, SnCn, and SnNiCn at a current density of 50 mA g⁻¹.

Fig. 11 shows the rate capability, cyclability, and coulombic efficiency data for the composites, with discharge capacity values calculated at different current densities, i.e., 50 to 1000 mA g⁻¹, for SnO, SnNi, SnCn, and SnNiCn samples. Fig. 11a shows the rate capabilities of all samples. The average reversible discharge capacities at 50, 100, 200, 400, 800, and 1000 mA g⁻¹ for SnNiCn are 5312, 1869, 1558, 1183, 980, and 919 mA h g⁻¹, respectively. When the current density returned to 50 mA g⁻¹, a capacity of 2166 mA h g⁻¹ was retained. This demonstrates the good rate performance and, also, the superiority of the SnO₂/Ni/CNT electrode with 4% Ni and 1% CNTs (SnNiCn) over pristine SnO₂ (SnO). The reversible discharge capacities at 50, 100, 200, 400, 800, and 1000 mA g⁻¹ for pristine SnO₂ were observed to be 1445, 330, 177, 93, 17, and 5, respectively. At the same time, values of 1564, 650, 497, 396, 296, and 231 mA h g⁻¹, respectively, were observed for SnNi (4% Ni), with values of 3098, 1188, 1015, 919, 823, and 774 mA h g⁻¹, respectively, being obtained for SnCn (1% CNTs). When the specific current returned to 50 mA g⁻¹, capacities of SnO, SnNi, and SnCn were 280, 524, and 1018 mA h g⁻¹, respectively. At 1000 mA g⁻¹, the pristine SnO₂ electrode delivers a very poor capacity, while the capacity of the SnO₂/Ni/CNT (SnNiCn) electrode shows a stable capacity, i.e., 919 mA h g⁻¹, at such a high current density. The enhanced capacity observed is quite obvious in the case of the SnO₂/Ni/CNT composite. The CNTs and Ni nanoparticles not only accommodate volume expansion due to pulverization but the metallic nanoparticles also act as an electrical conductor to facilitate electron transfer. This ultimately enhances the capacity of the nanocomposite. Fig. 11b shows cyclability data for all samples. To highlight the superiority of SnO₂/Ni/CNT with 4% Ni and 1% CNTs (SnNiCn) as an anode material for lithium ion batteries, we have performed cycling stability testing of all electrodes for up to 210 cycles at a current density of 400 mA g⁻¹. This clearly shows that the SnNiCn nanocomposite gives much better cycling performance compared with pristine SnO₂ (SnO) and the other SnNi and SnCn electrodes. Initially, the first two cycles were carried out at 50 mA g⁻¹ for activation, followed by eight cycles at 100 mA g⁻¹ and 200 cycles at 400 mA g⁻¹ to check the stability. SnO₂/CNT with 1% CNTs (SnCn) and SnO₂/Ni/CNT with 1% CNTs and 4% Ni (SnNiCn) both showed coulombic efficiencies (Fig. 10c) higher than 99.2% at 400 mA g⁻¹ with good capacity, but after 50 cycles the SnCn composite shows fading, with a capacity of 680 mA h g⁻¹, while SnNiCn shows a stable capacity of 1180 mA h g⁻¹, even after 210 cycles. The SnNi composite shows fading, with a capacity of 250 mA h g⁻¹, while pristine SnO₂ shows a capacity of 50 mA h g⁻¹ with fast fading. The stability of the capacity of SnNiCn might be due to the addition of an optimum concentration of Ni to the SnO₂/CNT nanocomposite. The addition of Ni increases the conductivity and decreases the lattice volume expansion, which facilitates the alloying and dealloying process. The Ni nanoparticles can successfully inhibit Sn coarsening during cycling, resulting in the enhancement of the stability during reversible conversion reactions. Also, Ni nanoparticles serve as an effective catalyst for the decomposition of Li₂O and the oxidation of metallic Sn from SnO₂ according to the following reactions:¹⁹

Ni + Li₂O → NiO + 2Li⁺ + 2e⁻ (7)

NiO + 2Li⁺ + 2e⁻ → Ni + Li₂O (8)

Fig. 11 (a) The rate performances at different current densities of all samples between 0.005 and 3 V. (b) The cycling performances of all samples at a current density of 400 mA g⁻¹ for 210 cycles. (c) The coulombic efficiencies of all samples at a current density of 400 mA g⁻¹ for 210 cycles in lithium-ion batteries.

This ultimately enhances the capacity, giving stability to the nanocomposite and improving performance. Also, an optimum amount of CNTs helps to support less degradation, because they act as a buffering agent against volume expansion and pulverization, and they can act as an electrical conductor to facilitate electron transfer involving nanosized SnO₂. In contrast, in pristine SnO₂ (i.e., without a Ni component or CNTs), huge volume expansion occurs during each cycle of lithiation and delithiation (Fig. S3†). For a more accurate analysis of the effects of the addition of CNTs and Ni, the rate performances of SnCn and SnNi samples mixed with 0.5% and 1.5% CNTs and 2 and 4% Ni were also studied and, based on the analysis of the test results, they showed better cycling performance than pristine SnO₂ but lower capacities at high current densities than the sample with optimum levels of CNTs and Ni, i.e., 1% CNT and 4% Ni, which might be due to the agglomeration of SnO₂.¹⁰

Conclusions

In summary, we have obtained pristine SnO₂ and SnO₂/Ni/CNT nanocomposites via a hydrothermal method. When SnO₂/Ni/CNT was explored for use in LIBs, it delivered a high specific capacity of around 2267 mA h g⁻¹ at 50 mA g⁻¹. Even at a high current density of 400 mA g⁻¹, the composite electrode exhibits a specific discharge capacity of 919 mA h g⁻¹ after 210 cycles. The improved electrochemical performance was attributed to the use of optimum Ni and CNT concentrations. Due to the addition of an optimum concentration of Ni into the SnO₂/CNT nanocomposite, the conductivity of the nanocomposite increases and the lattice volume expansion decreases, which facilitates the alloying and dealloying processes. Also, Ni nanoparticles can successfully inhibit Sn coarsening during cycling, resulting in the enhancement of stability during reversible conversion reactions. It is anticipated that such a detailed study of metal-oxide-based anodes for LIBs can provide a promising platform for battery applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Ms Anuradha would like to thank the Ministry of Electronics and Information Technology (MeitY), Department of Science and Technology, for financial support towards the implementation of the project (No. SR/WOS-A/CS-119/2018) under the woman scientist fellowship scheme. Also, the authors would like to thank C-MET Pune for providing research facilities and the nanocrystalline materials group for kind support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra01678d

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