Synergistic lithium storage of a multi-component Co₂SnO₄/Co₃O₄/Al₂O₃/C composite from a single-source precursor

Abstract

Endowing multi-component anode nanomaterials for lithium-ion batteries (LIBs) with integrated features for synergistically enhancing electrochemical performance is challenging via a simple preparation method. We herein describe an easy approach for preparing a multi-component Co₂SnO₄/Co₃O₄/Al₂O₃/C composite as the anode nanomaterial for LIBs, derived from a laurate anion-intercalated CoAlSn-layered double hydroxide (CoAlSn-LDH) single-source precursor. The resultant Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode delivers a highly enhanced reversible capacity of 1170 mA h g⁻¹ at 100 mA g⁻¹ after 100 cycles, compared with the bi-active composites designed without Al₂O₃ or carbon (Co₂SnO₄/Co₃O₄/C, Co₂SnO₄/Co₃O₄/Al₂O₃, and Co₂SnO₄/Co₃O₄) which are easily derived through the same protocol by choosing LDH precursors without Al cation or surfactant intercalation. The distinctly different cycling stability and rate capability of Co₂SnO₄/Co₃O₄/Al₂O₃/C among the different composite electrodes suggest that the high enhancement could result from the following synergistic features: the combined conversion and alloying reactions of bi-active Co₂SnO₄/Co₃O₄ during cycling, the buffering role of non-active Al₂O₃ and carbon, and the improved conductivity of the self-generated carbon matrix. The LDH precursor-based approach may be extended to the design and preparation of various multi-component transition metal oxide composite nanomaterials for synergistic lithium storage.

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1. Introduction

Rechargeable lithium-ion batteries (LIBs) have become the dominant power source for portable electronic devices and are the critical promising candidates for next-generation electric vehicles.^1–3 The rapidly widening market applications of LIBs increasingly inspire the development of electrode nanomaterials with improved capacity, long cycle life, high power density and energy density. Much effort has been devoted to introducing additional active components to improve transition metal oxide (TMO)-based nanostructures^4,5 as anode nanomaterials for LIBs, thereby alleviating the common problems of large voltage hysteresis and irreversible capacity. The materials studied range from single ternary TMOs (such as hollow porous CoFe₂O₄ nanocubes,⁶ Co₂SnO₄/graphene oxide,⁷ and porous hierarchical tremella-like NiMn₂O₄/C),⁸ to composite ternary TMOs^9–11 (CoO/CoFe₂O₄, Co₃O₄/Co₂MnO₄, and Mn₃O₄/Zn₂SnO₄). These nanomaterials are able to deliver enhanced reversible capacities owing to multi-component synergism. However, most TMOs still suffer from structural variation resulting from aggregation of nanoparticles into large particles during cycling, and from fast capacity decay due to poor conductivity,^12,13 especially for the exquisitely designed hierarchical micro/nanostructures.

Introduction of non-active components (such as Al₂O₃ or ZnO) into TMO composites is also crucial for enhancing cycling stability and rate performance. This design strategy has been successfully applied to electrode nanomaterials for LIBs, in which non-active components (such as Al₂O₃ or ZnO) can act as a buffering medium to effectively alleviate volume changes of the actives during the cycles and also to prevent aggregation of electrochemically active nanoparticles, thereby maintaining the structural integrity of the electrode nanomaterials.^14,15 ZnO-containing ZnO/ZnFe₂O₄ nanocubes were found to deliver a high capacity of 800 mA h g⁻¹ at 1000 mA g⁻¹ after 200 cycles, though they were derived by using toxic K₃Fe(CN)₆ with a hypertoxic product upon exposure to acidic or heated conditions.¹⁶ Also, a ZnMn₂O₄/ZnO/C nanohybrid exhibited a reversible capacity of 520 mA h g⁻¹ at 800 mA g⁻¹ after 100 cycles. It was synthesized by first ball-milling mixed precursors of ZnMnO₃ and acetylene black in absolute alcohol for 5 h, and subsequently by calcination at 600 °C under an argon atmosphere for 5 h.¹⁷ Similarly, a Al₂O₃-coated Fe₃O₄/reduced graphene oxide electrode achieved a reversible capacity of 717 mA h g⁻¹ after 200 cycles at a current density of 1/2C, and the thickness of the Al₂O₃ coating was elaborately controlled within a few nanometers using the atomic layer deposition (ALD) technique.¹⁸ The introduction of the non-active composites was, however, obtained typically using expensive or high energy-consumption equipment or complicated procedures. It is thus of great importance to develop carbon-supported active/non-active TMO composites by using simple, economic and scalable methods.

Layered double hydroxides (LDHs) are one type of anionic lamellar potential precursor material. LDHs are typically described by the general formula [M^II_1−xM^III_x(OH)₂]^x+(Aⁿ⁻)_x/n·yH₂O, where M^II and M^III are type-and-ratio-tunable cations occupying the octahedral holes in a brucite Mg(OH)₂-like layer, and Aⁿ⁻ is a type-tunable anion intercalated between the hydrated interlayer galleries. Thermal treatment of LDH precursors has been demonstrated to be an alternative method for the fabrication of various mixed metal oxide composites with well-dispersed multiple components.^19–22 We have recently demonstrated that C–Ni@NiO/Al₂O₃, derived from an NiAl–LDH/glucose mixture,²³ was able to deliver a stable charge capacity of 551 mA h g⁻¹ after 80 cycles at 100 mA g⁻¹, however, with a low reversible capacity. We hypothesized that the conductive carbon and the buffering by Al₂O₃ improved the cycling stability and rate capability.

In this study, we describe a multi-component Co₂SnO₄/Co₃O₄/Al₂O₃/C composite derived from a laurate anion (LA⁻)-intercalated ternary CoAlSn-LDH (CoAlSn-LA⁻-LDH) single-source precursor, via calcination and a further oxidation procedure (Fig. 1). The ternary elements Co, Al, and Sn were chosen for CoAlSn-LDH host to combine the conversion and alloying reactions of bi-active Co₂SnO₄/Co₃O₄ during cycling to improve electrochemical capacity.^24–26 Electrochemical evaluation shows that the resultant Co₂SnO₄/Co₃O₄/Al₂O₃/C composite indeed exhibits a highly enhanced electrochemical performance, in comparison with the different bi-active composites involving Co₂SnO₄/Co₃O₄, Co₂SnO₄/Co₃O₄/C, and Co₂SnO₄/Co₃O₄/Al₂O₃ which were obtained through the same protocol by choosing LDH precursors without Al cation or surfactant intercalation. The distinctly different cycling stability and rate capability of Co₂SnO₄/Co₃O₄/Al₂O₃/C among the different composite electrodes suggests that the high enhancement could result from the synergistic features.

Fig. 1 Schematic illustration of the formation of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite.

2. Experimental sections

2.1 Preparation

The CoAlSn-LA⁻-LDH precursor was synthesized via a conventional co-precipitation method. In brief, a mixture of Co(NO₃)₂·6H₂O (0.03 mol), Al(NO₃)₃·9H₂O (0.005 mol), and SnCl₄·4H₂O (0.005 mol) was dissolved in 100 mL distilled water, and then titrated by adding a solution of 100 mL NaOH (0.05 mol) and sodium laurate (0.015 mol) with stirring under a N₂ atmosphere at room temperature until pH = 10.5 ± 0.1. This process was followed by aging at 120 °C for 24 h. Afterwards, the precipitate obtained was centrifuged, washed with distilled water, and dried at 60 °C in a vacuum oven overnight.

Co₂SnO₄/Co₃O₄/Al₂O₃/C was obtained by calcination of the precursor in a fused quartz tube mounted in a programmable tube furnace at 700 °C for 200 min under a N₂ atmosphere with a temperature ramping rate of 2 °C min⁻¹. Then further calcination was employed in air at 400 °C for 120 min, followed by cooling to ambient temperature.

For comparison, Co₂SnO₄/Co₃O₄/Al₂O₃ was prepared using the same protocol from the CoAlSn-LDH precursor without the intercalation of laurate anions, and Co₂SnO₄/Co₃O₄/C and Co₂SnO₄/Co₃O₄ were obtained from the CoSn-LDH precursor with and without the intercalation of laurate anions, respectively.

2.2 Characterization

Powder XRD measurements were performed on a Rigaku XRD-6000 diffractometer, using Kα radiation (40 kV, 30 mA, λ = 0.1542 nm) in the 2θ range from 3 to 70°. The morphology and chemical composition of the samples were investigated by using scanning electron microscopy (SEM, ZEISS Supra 55). The accelerating voltage was 20 kV. Transmission electron microscopy (TEM, JEM JEOL 2100) was used to visualize the morphologies, sizes and structures of the products. High-resolution transmission electron microscopy (HRTEM) images were recorded on the JEOL JEM-2100 microscope. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging and EDX mapping were performed on a Tecnai G2 F30 S-TWIN equipped with energy dispersive X-ray (EDX) detection. Room-temperature FTIR spectra were recorded in the range 400–4000 cm⁻¹ with a resolution of 2 cm⁻¹ on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). Raman spectroscopy was recorded on a confocal Raman spectrometer (Renishaw RM2000) with a 532 nm excitation laser (laser spot size of 0.5 μm) operated at a low power level (∼2 mW). Samples were drop-cast onto a silicon substrate for measurements. X-ray photoelectron spectroscopy (XPS) (Thermo VG Scientific) was recorded under high vacuum on samples dried onto silicon wafers using Al Kα 1486.6 eV radiation at 400 W (15 kV).

2.3 Electrochemical measurements

Electrochemical measurements were performed with 2032 coin cells assembled in an Ar-filled glove box. Active material, ketjen black, and poly(vinyl difluoride) (PVDF) were mixed with a 70 : 20 : 10 weight ratio, and pasted on a Cu foil, thereby preparing working electrodes. The loading mass of composite material was about 10 mg cm⁻². Lithium foil was used as the counter electrode, and a glass fiber (GF/D, from Whatman) as a separator. The electrolyte used consisted of a solution of 1 M LiPF₆ salt in ethylene carbonate (EC)/dimethyl carbonate (DMC)/(1 : 1 by wt%) (obtained from Tianjing Jinniu Power Sources Material Co. Ltd.). Galvanostatic cycling and the measurement of the charge–discharge profiles of the assembled cells were carried out using a LAND CT2001A battery tester in the voltage range from 0.01 to 3.0 V (vs. Li⁺/Li). Cyclic voltammetry (CV) was conducted on an electrochemistry workstation (CHI 660E) at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0–3.0 V (vs. Li⁺/Li). EIS measurements were performed over the frequency range between 100 KHz and 0.1 Hz at open circuit potential using a Parstat 2273 advanced electrochemical system.

3. Results and discussion

The CoAlSn-LA⁻-LDH precursor was prepared by using a conventional co-precipitation method. XRD results show that a shift of the (003) basal peak was clearly visible for the resulting precursor at an angle 2θ < 10° (Fig. 2a), which is lower than that of the pristine CoAlSn-CO₃²⁻-LDH (2θ = 11.6°; Fig. S1a†). This pronounced shift strongly suggests the intercalation of the surfactant into the interlayer galleries.²⁷ After the completion of calcination of the precursor, the obtained product shows the XRD characteristics of Co₂SnO₄ and Co₃O₄ (Fig. 2b). The diffraction peaks observed at 2θ values of 17.8, 29.2, 34.4, 36, 42, 45.7, 51.8, 55.2, 60.6 and 63.7° can be well indexed to the (111), (220), (311), (222), (400), (331), (422), (511), (440), and (531) planes of Co₂SnO₄ (JCPDS 29-0514), respectively, and the other diffraction peaks can be indexed to Co₃O₄ (JCPDS 43-1003). A halo was discernible with an angular range from 20° to 30°, attributable to the amorphous carbonaceous material formed during the carbonization process. Additionally, no characteristic peak was visible for Al₂O₃, indicative of the amorphous Al₂O₃ phase. This amorphous feature is indeed consistent with the previous studies of our and others' laboratories concerning Al₂O₃-containing composites derived from surfactant-intercalated LDHs.^23,28,29 For comparison, the other three bi-active Co₂SnO₄/Co₃O₄ composites were easily obtained by varying the cation type of the LDH precursors. A comparison of the XRD patterns of all the bi-active composites clearly shows the consistency of bi-active-component basal reflection peaks (as shown in Fig. 2b and S1b†), thus strongly suggesting the feasibility and good reproducibility of the preparation method.

Fig. 2 (a) XRD patterns of the CoAlSn-LA⁻-LDH precursor, (b) XRD patterns, as well as (c) SEM and (d) HRTEM images of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite.

The surface morphology was observed for the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite using SEM. Fig. 2c displays the particle-like morphology of the composite. The small dimensional sizes can be assigned to the confinement from both the net trap effect of Al₂O₃ and the carbon formed via carbonization during the high-temperature calcination.²³ TEM visualization confirms the formation of carbon-supported nanoparticles (Fig. S2†). A similar size distribution was also observed for the Co₂SnO₄/Co₃O₄/C composite with carbon (Fig. S3a†), which was derived from the CoSn-LA⁻-LDH precursor without the Al cation. High-resolution TEM of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite provides further evidence for the co-existence of carbon-supported bi- and non-active Co₂SnO₄/Co₃O₄/Al₂O₃ (Fig. 2d). Two types of highly crystalline regions were resolved, with d-spacing values of 0.49 and 0.3 nm corresponding to the (111) and (220) planes of cubic Co₂SnO₄, respectively, and values of 0.24 and 0.28 nm respectively corresponding to the (311) and (220) planes of cubic Co₃O₄. The planes involved are consistent with the above-mentioned XRD patterns of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite. One domain with curved and bent graphene-like sheets can be ascribed to carbon. It is noteworthy that the bi-active composites without carbon, i.e., Co₂SnO₄/Co₃O₄/Al₂O₃ and Co₂SnO₄/Co₃O₄, were both found to be more homogenous in dimensional size distribution, but to display larger dimensional sizes and crystalline sizes, as shown in SEM (Fig. S3b and c†) and TEM images (Fig. S3e and f†), when compared with both bi-active composites with carbon (Fig. 2c and d, and S3a and d†). A dotted-line-circle marked domain could be identified as amorphous Al₂O₃ or carbon, owing to the limited instrumental resolution (Fig. 2d). We additionally carried out HAADF-STEM imaging and EDX mapping on the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite. The resulting HAADF-STEM/EDX observations (Fig. S4†) show the nanoparticle-like morphology of the composite. The major elements Co and O, as well as the minor elements Al and Sn, are well dispersed within the composite nanoparticles, in agreement with the molar ratios of the LDH precursor.

Fig. 3 (a) FT-IR spectra of Co₂SnO₄/Co₃O₄/Al₂O₃/C, Co₂SnO₄/Co₃O₄, Co₂SnO₄/Co₃O₄/Al₂O₃, and Co₂SnO₄/Co₃O₄/C composites. (b) Raman spectrum of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite.

FT-IR spectra were recorded for Co₂SnO₄/Co₃O₄/Al₂O₃/C and the other three composites. Fig. 3a shows that all the composites exhibit similar absorption bands of each component contained. Two absorption peaks were identified at 447 and 578 cm⁻¹, corresponding to the stretching vibration of Co²⁺–O and Co³⁺–O bonds, respectively. The absorption band at 667 cm⁻¹ can be assigned to the asymmetric vibration of the Sn–O–Sn bond, and the one at 750 cm⁻¹ to the stretching vibration of the Al–O bond.^30,31 The broad and strong absorption band visible around 3435 cm⁻¹ was ascribed to the symmetric stretching vibration of the –OH group owing to the presence of adsorbed moisture.²⁶ In the case of the composites with carbon, two absorption peaks were resolved at 1580 and 1635 cm⁻¹, ascribed to the stretching vibration of C=C and the stretching vibration of C=O, respectively, and the peaks at 1384 and 1081 cm⁻¹ were ascribed to the stretching vibration absorption of the C–O bond. Absorption peaks at 2924 and 2972 cm⁻¹ confirm the symmetric and asymmetric stretching vibrations of the C–H bond, respectively.^32,33 These results elucidate the formation of carbon, which is converted from the long alkyl chains of laurate anions during the thermal treatment. Raman analysis further manifests the nature of the carbon support of Co₂SnO₄/Co₃O₄/Al₂O₃/C (Fig. 3b). Two broad peaks at 1340 cm⁻¹ (D-band) and 1586 cm⁻¹ (G-band) are clearly visible, corresponding to the disorder induced along the c-axis of graphitic carbonaceous material and to the vibration of sp² hybridized carbon atoms in graphitic carbon,³⁴ respectively. The calculated I_D/I_G ratio is 1.15, indicative of the highly disordered feature of the carbon matrix.³⁵

Chemical states were characterized for the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite by the XPS technique (Fig. 4). The high-resolution of the Co 2p spectrum shows two obvious spin orbit doublet peaks centred at 781.5 and 797.0 eV, accompanied by two prominent shake-up satellite peaks at 785.8 and 803.0 eV, respectively. These four peaks can be ascribed to Co 2p_3/2 and Co 2p_1/2 orbits,^36,37 obviously verifying the presence of Co²⁺. Another two major peaks centred at 779.7 and 795.3 eV can be attributed to Co 2p_3/2 and Co 2p_1/2 in the Co₃O₄ phase, respectively.³⁸ Fig. 4b shows that the Sn 3d spectrum can be fitted by two strong energy bands centred at 486.3 and 494.7 eV, which are ascribed to the 3d_5/2 and 3d_3/2 orbits, respectively.^36,37 In addition, the Al 2p spectrum shows one major peak at 74.5 eV (Fig. 4c), indicative of the formation of the amorphous Al₂O₃ phase.^23,39 Also, the C 1s XPS spectrum could be fitted to two strong peaks at 284.9 and 288.9 eV (Fig. 4d), which can be ascribed to the presence of the C–C and C–O of the supporting carbon matrix.⁴⁰ Comparison of the Co 2p and Sn 3d spectra between these four composites (Fig. S5†) reveals that cobalt cations are present in the valent states of Co²⁺ and Co³⁺, and that the tin cation is in the valent state of Sn⁴⁺, again confirming the consistent reproducibility in preparing the bi-active Co₂SnO₄/Co₃O₄ composites through the protocol.

Fig. 4 XPS spectra of Co₂SnO₄/Co₃O₄/Al₂O₃/C composite for (a) Co 2p, (b) Sn 3d, (c) Al 2p and (d) C 1s.

Furthermore, the contents of Co, Al and Sn of the nanomaterial were determined on the basis of the XPS data. The Co₂SnO₄/Co₃O₄/Al₂O₃ mass ratio was calculated to be 48.6 : 44.5 : 6.9. The molar ratio of Co/(Al + Sn) was thus determined to be 2.85, quite close to that (ca. 3.0) of the starting materials used. Element analysis gave rise to a 6.1 wt% content of carbon. From various early studies,^23,28,29 the multiple components, including nanosized bi-active Co₂SnO₄/Co₃O₄, the low-content of non-active Al₂O₃, and the self-generated conductive carbon, are expected to synergistically facilitate the reversible specific capacity and cycling stability of the Co₂SnO₄/Co₃O₄/Al₂O₃/C nanocomposite when it is used as an anode nanomaterial for LIBs.

Cyclic voltammetry (CV) curves were recorded for the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode. Fig. 5a shows that a strong reduction peak was found at 0.55 V (vs. Li⁺/Li) in the first cycle in the cathodic process, attributable to the formation of a solid electrolyte interface (SEI) and Li₂O, and the decomposition of Co₂SnO₄ and Co₃O₄,^24,41,42 as shown by the following Reactions (1) and (2). Another reduction peak was observed at 1.0 V (vs. Li⁺/Li), corresponding to the formation of Li–Sn alloys, as accounted for by Reaction (3). In the anodic process, two broad oxidation peaks were clearly visible at 0.5 and 2.1 V, typically assigned to the de-alloying process (inverse response of Reaction (3)) and the re-oxidation reaction of cobalt and tin atoms (as expressed by Reactions (4) and (5)), respectively.²⁴ In the second and subsequent cycles, the cathodic peaks were found to shift remarkably to 1.2 V and 1.4 V, whereas no obvious shift was observed for the anodic process, as also reported in previous studies.^24,26,41 On the basis of the above CV curves, together with the storage mechanisms of Co₂SnO₄ and Co₃O₄ that have been widely reported previously,^7,24,25,42 a combined reaction mechanism, involving intercalation and conversion reactions, is thought to occur between Co₂SnO₄ and Co₃O₄ with Li⁺. The electrochemical reactions for the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite can thus be summarized by the following reactions:

Co₂SnO₄ + 8Li⁺ + 8e⁻ → 2Co + Sn + 4Li₂O (1)

Co₃O₄ + 8Li⁺ + 8e⁻ ↔ 3Co + 4Li₂O (2)

Sn + 4.4Li ↔ Li_4.4Sn (3)

Sn + 2Li₂O ↔ SnO₂ + 4Li (4)

Co + Li₂O ↔ CoO + 2Li (5)

Fig. 5 (a) The first three cyclic voltammetry curves from 0–3.0 V at 0.1 mV s⁻¹, (b) the first three charge–discharge profiles at 100 mA g⁻¹ of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite, (c) cycling behaviour of the charge capacity at a rate of 100 mA g⁻¹, and (d) rate capacity from 100, through 200, 400, 800, 1000, 2000, to 100 mA g⁻¹ of Co₂SnO₄/Co₃O₄/Al₂O₃/C, Co₂SnO₄/Co₃O₄, Co₂SnO₄/Co₃O₄/Al₂O₃ and Co₂SnO₄/Co₃O₄/C composites.

Galvanostatic discharge/charge tests were performed for the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode between 0.01 and 3 V at 100 mA g⁻¹ (Fig. 5b). It is noteworthy that the initial capacity (1515 mA h g⁻¹) of the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode is slightly higher than that (1420 mA h g⁻¹) of the Co₂SnO₄/Co₃O₄/C electrode. However, the theoretical capacity of the former with Al₂O₃ was calculated to be 935 mA h g⁻¹, which is slightly lower than that (1053 mA h g⁻¹) of the latter without Al₂O₃. This discrepancy strongly suggests an enhancement in initial capacity of the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode, which can be ascribed to the nanosized effect⁴³ of the Co₂SnO₄/Co₃O₄/Al₂O₃/C composite. As observed above by TEM, the Co₂SnO₄/Co₃O₄/Al₂O₃/C nanoparticles show a slightly smaller mean dimensional size (ca. 8.2 nm) than that (ca. 12.5 nm) of the Co₂SnO₄/Co₃O₄/C nanoparticles, which can be ascribed to the confinements from the net trap effect²³ of the non-active Al of the CoAlSn-LDH precursor, and from the carbon formed. In a previous study, metal cations within the LDH layers have been demonstrated to adopt a fully ordered arrangement.⁴⁴ During thermal decomposition of the LDH precursors, the symbiotic non-active Al₂O₃ component serves as a well-dispersed confining domain to prevent the growth and agglomeration of active Co₂SnO₄/Co₃O₄ nanoparticles. Therefore, the nanosized Co₂SnO₄/Co₃O₄/Al₂O₃/C composite is able to exhibit the improved initial reversible capacity in the first cycle. In addition, a low initial coulombic efficiency was determined (51.2%). The capacity loss can be generally attributed to the formation of an SEI film during the first charge.⁴⁵ Although a close similarity in the first three discharge/charge profiles was found between the electrodes of Co₂SnO₄/Co₃O₄/Al₂O₃/C and the other three composites (Fig. S6†), further comparison reveals a remarkable difference in cycling performance between the Co₂SnO₄/Co₃O₄/Al₂O₃/C and the other electrodes. Over 100 cycles, the coulombic efficiencies of Co₂SnO₄/Co₃O₄/Al₂O₃/C were found to remain stable (Fig. S7†). After the 100^th cycling, the reversible charge capacity of Co₂SnO₄/Co₃O₄/Al₂O₃/C was 1170 mA h g⁻¹, higher than the theoretical values of Co₂SnO₄ (1105 mA h g⁻¹) and Co₃O₄ (890 mA h g⁻¹). However, the other three electrodes exhibited a rapid decay in reversible capacity during the cycling processes. The enhanced reversible capacity could be typically attributed to the synergistic effect of the above-mentioned multiphase reaction between bi-active Co₂SnO₄ and Co₃O₄, as observed for transition metal oxide composites.^{11,24,26,46,47} Further comparison of reversible specific capacity between our Co₂SnO₄/Co₃O₄/Al₂O₃/C and the Co₂SnO₄- or Co₃O₄-based electrodes reported previously^{7,24,42,43,47–50} is summarized in Table S1.† We found that the reversible charge capacity of our composite electrode may be comparable to these other electrodes when considering the approximate mass weights of the active materials used.

From various early studies,^{16–18,51–53} it well known that an active component reacts with lithium, while a non-active component acts as an electrochemically inert matrix and buffers the volume change during the charge/discharge cycle, thereby resulting in remarkable improvements in the cycle life of composite nanomaterials. To understand the role of non-active Al₂O₃, a further comparison of cycling performance was conducted between the composite electrodes with and without Al₂O_3. We can see that the Co₂SnO₄/Co₃O₄/C electrode without Al₂O₃ was able to show a reversible capacity comparable to that of the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode during the first 15 cycles, but afterwards the reversible capacity dropped rapidly, resulting in a notably lower capacity retention of 49.5% in comparison to Co₂SnO₄/Co₃O₄/Al₂O₃/C (77.8%). This strongly suggests the necessity of the buffering effect of non-active Al₂O₃. The buffering role can also be supported by comparing the Co₂SnO₄/Co₃O₄ and Co₂SnO₄/Co₃O₄/Al₂O₃ electrodes, viz. with and without Al₂O₃. By the 100th cycle, we found that low charge capacities were 388 and 459 mA h g⁻¹ for Co₂SnO₄/Co₃O₄ and Co₂SnO₄/Co₃O₄/Al₂O₃, respectively. This appreciable difference clearly reveals that the Co₂SnO₄/Co₃O₄/Al₂O₃ is indeed capable of delivering a slightly improved cycling performance owing to the buffering effect of non-active Al₂O₃. This inactive component strategy has previously been applied to electrodes for LIBs, typically involving a several-nanometer-thick ultrathin coating of Al₂O₃ on LiCoO₂ achieved by an expensive ALD technique⁵¹ or exquisite control of the pH value of the buffer solution,⁵² or by using an Al₂O₃ matrix to support a Cu₂Sb–Al₂O₃–C composite synthesized via the high-energy mechanical reduction of Sb₂O₃ with Al and Cu metals in the presence of carbon (acetylene black).⁵³ Considering the unique flexibility offered by varying the type and stoichiometric ratio of LDH cations over a wide range, our LDH precursor-derived preparation method could be relatively simple and suitable for producing large quantities of multi-component nanomaterials for practical application.

Furthermore, we can see from Fig. 5c that both electrodes with carbon (Co₂SnO₄/Co₃O₄/Al₂O₃/C and Co₂SnO₄/Co₃O₄/C) exhibit highly enhanced reversible capacities and cycling stabilities, compared with the two electrodes without carbon (Co₂SnO₄/Co₃O₄/Al₂O₃ and Co₂SnO₄/Co₃O₄). Obviously, the supporting carbon plays a critical role in boosting conductivity and further electrochemical performance, as is well-recognized from various previous studies of carbon-supported anode nanomaterials.^54,55

Rate capability was examined for the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode and the other three electrodes for comparison. Fig. 5d shows that the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode exhibits reversible capacities of 1420, 1285, 1110, 920, 770, and 584 mA h g⁻¹ at the current densities of 100, 200, 400, 800, 1000, and 2000 mA g⁻¹. The reversible specific capacity was found to return to 1245 mA h g⁻¹ when the current density was reset to 100 mA g⁻¹, suggesting that the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode remains very stable during cycling. However, the other three electrodes exhibited much lower rate capacities at the corresponding current densities in comparison with the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode. On the basis of the distinctly enhanced electrochemical performance of Co₂SnO₄/Co₃O₄/Al₂O₃/C among all the composite electrodes, we thus believe that the high enhancement of the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode results from multiple-component synergistic contributions: (i) combined conversion and alloying reactions of nanosized bi-active Co₂SnO₄/Co₃O₄ during cycling; (ii) buffering by Al₂O₃ and carbon, which are well-known to accommodate strain change and prevent aggregation of active components; and (iii) conductive carbon which improves the conductivity of the electrode and thereby rapid electronic transport.

Electrochemical impedance spectra (EIS) were recorded for the four electrodes to provide further evidence for the improved conductivity. Fig. 6 shows that the impedance spectra consist of a high-to-medium-frequency depressed semicircle and a low-frequency sloping linear line. Basically, the high frequency semicircle is attributed to SEI resistance and contact resistance, the medium-frequency semicircle is assigned to the charge-transfer impedance of the electrode reaction at the electrode/electrolyte interface, and the low-frequency sloping linear tail corresponds to the Warburg impedance that reflects the solid-state diffusion of lithium ions within the bulk electrode.^56–59 The impedance spectra of both electrodes with carbon exhibit smaller semicircle diameters compared with those of the electrodes without carbon. The values of the semicircle diameters of the Co₂SnO₄/Co₃O₄/Al₂O₃/C and Co₂SnO₄/Co₃O₄/C composites were determined to be 36.9 and 36.7 Ω, respectively, which are much lower than those of Co₂SnO₄/Co₃O₄ and Co₂SnO₄/Co₃O₄/Al₂O₃ (60.9 and 86.3 Ω, respectively). In addition, it is noteworthy that both electrodes with carbon show smaller Warburg impedances when compared with those for the electrodes without carbon. These decreases clearly suggest that the electrodes with carbon possess improved electrical conductivity and deliver an enhancement effect in the kinetics for lithium storage. Although the Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode contains the low-mass-weight non-active Al₂O₃, the electrode is able to exhibit low impedance quite close to that of the Co₂SnO₄/Co₃O₄/C electrode, and eventually deliver the highly improved electrochemical performance, as a result of synergistic contributions from the conductive carbon, bi- and non-active components.

Fig. 6 Comparison of Nyquist plots for the electrodes of Co₂SnO₄/Co₃O₄/Al₂O₃/C, Co₂SnO₄/Co₃O₄, Co₂SnO₄/Co₃O₄/Al₂O₃ and Co₂SnO₄/Co₃O₄/C.

4. Conclusions

We have demonstrated an easy method to prepare a multi-component Co₂SnO₄/Co₃O₄/Al₂O₃/C composite as an efficient anode nanomaterial for LIBs. The Co₂SnO₄/Co₃O₄/Al₂O₃/C electrode is able to exhibit a highly enhanced reversible capacity of 1170 mA h g⁻¹ after 100 cycles, compared with the designed bi-active Co₂SnO₄/Co₃O₄ composites without Al₂O₃ or carbon (Co₂SnO₄/Co₃O₄/C, Co₂SnO₄/Co₃O₄/Al₂O₃, and Co₂SnO₄/Co₃O₄) prepared by tuning the cation types of the LDH precursors through the same protocol. The componential and nanostructural features are capable of facilitating the reversible capacity with synergistic contributions from bi-active Co₂SnO₄/Co₃O₄ and multi-electron reactions during the cycle processes, high cycling and rate stabilities from the buffering role of the non-active Al₂O₃ and carbon, and the improved conductivity of the carbon matrix. Our LDH precursor-based approach may be extended to the preparation of other carbon-supported metal oxides as lithium-storage hybrid nanomaterials, on the basis of the great versatility created by varying the metal cations and surfactant interlayer anions of LDH precursors.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program 2014CB932102) and the National Natural Science Foundation of China, as well as the Program for Changjiang Scholars and Innovative Research Team in University (IRT1205).

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Footnote

† Electronic supplementary information (ESI) available: XRD, TEM, XPS, and charge–discharge profiles. See DOI: 10.1039/c5ra09607c

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