Simple synthesis of MoO₂/carbon aerogel anodes for high performance lithium ion batteries from seaweed biomass

Abstract

The sluggish kinetics and large volume expansion occurring during cycle process are critical disadvantages for molybdenum dioxide (MoO₂) based anode materials for lithium-ion batteries (LIBs). The design and controllable synthesis of MoO₂ based materials with unique hierarchical structure are desired. In this work, we present a one step in situ method to synthesize MoO₂/carbon aerogel composites with MoO₂ nanoparticles embedded in the carbon aerogel matrix using an environmentally friendly alginate as the carbon precursor. The key feature of our strategy involves the use of a carbon aerogel matrix which facilitates the Li ion transport and offers the space for MoO₂ volume expansion during the discharge and charge processes. The composite prepared under optimum conditions, namely a stabilization temperature at 375 °C, exhibited a brilliant performance with a specific capacity of 574 mA h g⁻¹ at a current density of 100 mA g⁻¹, good cycle stability (i.e., a reversible capacity of 490 mA h g⁻¹ at a current density of 200 mA g⁻¹ for 120 cycles), excellent rate capability (remains at 331 mA h g⁻¹ even at a current density of 1000 mA g⁻¹). This finding presents an easy, eco-friendly and efficient strategy for the fabrication of potential high-performance LIBs anodes.

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Introduction

In view of the growing need for next-generation high-power applications (e.g., portable electronics, renewable energy storage, electric vehicles and smart grids), seeking high capacity and stable materials for lithium-ion batteries (LIBs) has become a most demanding task. Molybdenum dioxide (MoO₂) has attracted much attention due to its high theoretical capacity of 838 mA h g⁻¹ (5360 mA h cm⁻³), superb electronic conductivity (>1 × 10⁴ S cm⁻¹), and facile ion transport properties.^1,2 However, the sluggish kinetics of bulk MoO₂ and the large volume expansion occurring during the Li ion insertion/extraction process, causes the breakdown of capacity delivery and detachment of the active material from the current collector, thus leading to rapid capacity decay.³ A variety of approaches have been taken to solve this drawback, including the synthesis of nanostructure MoO₂ (ref. 4 and 5) and designing MoO₂/carbon composite,^6–8 Among the previous research works, the method can be briefly divided into two types: (1) MoO₂ nanoparticle or MoO₂/carbon composite with different size or morphology can be directly achieved by hydrothermal synthesis method;^9–11 (2) a two-steps process. MoO₃ nanostructure was first synthesized by hydrothermal synthesis method, and subsequently was mixed with various carbon precursors (e.g., glucose and oleic acid) to form MoO₂/carbon composite by thermal treatment.^12,13 These synthesis process was either complex or toxic. Therefore, it is very crucial to design a synthesis method which is not only easy and efficient but also environment-friendly.

Alginate, as a common seaweed biomass, has been applied to the synthesis of LIBs anode materials as a carbon matrix because it can chelate with various metal cations such as Ca²⁺, Fe²⁺, Co²⁺, Ni²⁺, Mn²⁺, Sn²⁺ to form the so-called “egg-box model”.^14–19 Our research team has made several inspiring discoveries in this field.^20–23 Lv et al. fabricated high performance fibrous transition metal oxides anodes such as elemental Ni doped NiO, yolk–shell structured carbon@Fe₂O₃ and hollow CuO fibers with controllable nanostructures for LIBs by using 1 dimensional seaweed fibers as templates.²² Li et al. prepared carbonaceous microfibers embedded with Co₃O₄ nanoparticles by direct pyrolysis of wetspun cobalt alginate fibers. The synthesized Co₃O₄-carbonaceous microfibers affords a high reversible capacity (780 mA h g⁻¹) for long cycling when used as the anode material for LIBs.²³

In this work, we described a one step in situ method to synthesize a MoO₂/carbon aerogels (MoO₂@SAC) with MoO₂ nanoparticles embedded in carbon aerogels matrix using an environmental friendly sodium alginate (SA) as the carbon precursor. A hydrogel networks prepared via an ion-exchange process by replacing Na⁺ in SA with H⁺ is used to wrap the ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O (AMM)). Meanwhile, the hydrogel also provide an appropriate pH environment for AMM to transform into MoO₃.²⁴ Alginate acid support as the carbon precursor to provide conductive aerogels matrix. Furthermore, we believed that different Mo-contained bimetallic oxides such as Fe₂Mo₃O₈, CaMoO₄ can be obtained by this method using different metal cation solution substituting for hydrochloric acid.^25,26

The synthesis of the MoO₂@SAC involved a three-step process, which was illustrated in Fig. 1. Firstly, SA aqueous solution mixed with AMM was dropped into HCl solution under stirring to fulfil a complete exchange between H⁺ and Na⁺ to get the AMM hydrogels (Fig. 1a). Subsequently, the obtained hydrogels were dehydrated via a freeze-drying process to generate the AMM aerogels (Fig. 1b). Finally, the AMM aerogels were stabilized in air, and subsequently carbonized under nitrogen atmosphere. During the air stabilization process, AMM was split into an intermediate product MoO₃ (eqn (1)) (Fig. S1†), while in the carbonization process MoO₃ would be reduced to MoO₂ (eqn (2)) and the product MoO₂@SAC-X (X = 350, 375, 400, denotes as the different stabilization temperature) aerogels were achieved (Fig. 1c).

(NH₄)₆Mo₇O₂₄·4H₂O → 6NH₃ + 7MoO₃ + 7H₂O (1)

MoO₃ + C → MoO₂ + CO₂ (2)

Fig. 1 Schematic illustration for scalable synthesis of MoO₂@SAC aerogels.

Results and discussion

Fig. 2a shows the X-ray diffraction (XRD) patterns of AMM and MoO₂@SAC-X aerogels, respectively. For AMM aerogels, the broad weak peak at a 2θ = 21.0° is ascribed to a typical “egg-box” structure in α-L-guluronate (G) junction zones where not mental ions but H⁺ exchanging with Na⁺, which leads to the low crystallinity.²³ For MoO₂@SAC-X aerogels, sharp diffraction peaks at 2θ values of 25.9, 31.5, 35.1, 41.3 and 53.5 are observed, which corresponding to the diffraction of (110), (−102), (020), (−212) and (022) planes of monoclinic MoO₂ with a space group of P₂₁/c₍₁₄₎ (JCPDS 032-671). It indicates that Mo₇O₂₄⁶⁻ cations in AMM aerogels completely converted to MoO₂ species after the thermal process. It should be point that under the stabilization procedure (350–400 °C), AMM was split into an intermediate product MoO₃. After the subsequent carbonization process, no diffraction peaks corresponding to MoO₃ are observed, indicating that MoO₃ was completely reduced. XRD patterns also confirm MoO₂@SAC-X crystalline strengthens with stabilization temperature increases. The XRD patterns of AMM aerogels after stabilization at 350, 375 and 400 °C are shown in Fig. S1.† All the diffraction peaks are indexed to MoO₃, and the crystalline enhances with stabilization temperature increases. Based on our analysis, incomplete decomposition of Mo₇O₁₄⁶⁻ lead to poor crystallization of MoO₃ for MoO₂@SAC-350 than that of MoO₂@SAC-400. During the subsequent carbonization process, the undecomposed Mo₇O₁₄⁶⁻ proceeds to generate MoO₃ which is then reduced to MoO₂. The thermal decomposition reaction in carbonization process affect crystal growth of MoO₃, which may lead to the poor crystalline. However, MoO₂@SAC aerogels are also synthesized by annealing the AMM aerogels at 600 °C under N₂ without a stabilization process. XRD pattern indicates all the sharp diffraction peaks are attributed to MoO₂. (Fig. S2†) It is worth noting that the crystalline of MoO₂@SAC is higher than MoO₂@SAC-350 but lower than those of MoO₂@SAC-375 and 400. The results indicate that direct reduction of AMM aerogels is unfavorable to the crystallization course for MoO₂, which may further influence the product morphology and properties.

Fig. 2 (a) XRD patterns of MoO₂@SAC-X and AMM aerogels, (b) TGA curves of MoO₂@SAC-X aerogels, (c) Raman spectrum of MoO₂@SAC-375.

Thermogravimetric analysis (TGA) was carried out under air to evaluate the amount of carbon in MoO₂@SAC aerogels. As shown in Fig. 2b, the mass loss occurred before 200 °C is mainly due to the volatilization of absorbed H₂O or other volatile impurities on the sample surface. Fig. S3† shows that the weight loss for SAC aerogels (synthesized by pyrolysis of alginic acid aerogels) started at around 350 °C and ended at about 500 °C without any residue. For the MoO₂@SAC-X aerogels, it can be seen that the weight loss started at 350 °C and ended at 530 °C. The residual loss after 530 °C was about 45%, 63% and 81%, respectively, corresponding to the oxidation product MoO₃. The theoretical value of the weight increase from MoO₂ to MoO₃ is 12.5 wt%, thus, the contents of carbon in MoO₂@SAC-X aerogels can be roughly calculated to be around 66.7%, 43.7% and 36.0%. The result demonstrates that more carbon is consumed as the temperature increased in the stabilization process.

Fig. 2c shows Raman spectrum of MoO₂@SAC-375 aerogels. The peaks at 816, 735 and 567 cm⁻¹ can be attributed to ν(O–Mo–O), ν(O₂–Mo) and ν(O₁–Mo), respectively, confirming the bond vibration modes of MoO₂. The finger bands at 355 and 197 cm⁻¹ can be assigned to the phonon vibration modes of MoO₂.^27,28 The D band (1350 cm⁻¹) and the G band (1592 cm⁻¹) corresponding to disordered carbon and graphite bands were both observed.²⁹ Besides, the ratio of I_D/I_G is about 7/11, indicating a high degree of graphitization in MoO₂@SAC-375.

The morphology of MoO₂@SAC-X aerogels are investigated by the field emission scanning electron microscopy (FE-SEM). Fig. 3a and b show the SEM images of MoO₂@SAC-375 aerogels. Obviously, the aerogels are composed of MoO₂ nanoparticles which are embedded in carbon aerogels matrix. The nanosized MoO₂ is beneficial to the transition of Li ions and provides more active sites. What is more, the carbon layer outside can enhance the conductivity and accommodate the volume change of MoO₂ nanoparticles during charge and discharge process. The morphology of MoO₂@SAC-350 and MoO₂@SAC-400 are also displayed in Fig. S4.† It shows that no MoO₂ nanoparticle is visible when the stabilization temperature is 350 °C. When the temperature rose to 400 °C, aggregation phenomenon occurred and the carbon layer outside MoO₂ nanoparticles disappeared at the same time. These morphology change of various stabilization temperature may lead to a decrease of Brunauer–Emmett–Teller (BET) surface and further influence the electrochemical performance.

Fig. 3 (a and b) SEM images of MoO₂@SAC-375. (b) Image showing a magnified view of the area enclosed by the red box in (a). (c) Nitrogen adsorption–desorption isotherms and (d) the pore diameter distribution of MoO₂@SAC-375.

The BET surface areas, pore volume and average pore size of SAC and MoO₂@SAC-X aerogels are shown in Table S1 and Fig. S7.† SAC aerogels show a high BET surface area of 360 m² g⁻¹, owing to its porous network structure. For MoO₂@SAC-X aerogels, the BET surface area is 210.1, 196.7 and 25.22 m² g⁻¹, respectively. The BET surface areas decreased because more carbon participated in the redox reaction and the carbon network partly destroyed with stabilization temperature increased. The high surface area and porosity are attributed to reaction between MoO₃ and carbon, and the thermal decomposition of alginic acid, where the oxygen-containing groups (–OH and –COOH) convert to H₂O and CO₂.²³ The N₂ adsorption–desorption isotherm and Barret–Joyner–Halenda (BJH) pore size distribution analysis of MoO₂@SAC-375 are displayed in Fig. 3c and d. It displays a typical type I adsorption isotherm and the average BJH pore diameter calculated from the adsorption branch of the isotherm is mainly centred at 5 nm, indicating the mesoporous structure. The large specific surface area provides more active sites for Li ions storage. In addition, the porous structure is not only conductive to the transition of Li ions through the active material, but also beneficial to the accommodation of volume expansion during discharge/charge process.

To further examine the architecture, the MoO₂@SAC-375 aerogels was investigated by TEM and HRTEM. Fig. 4a shows MoO₂ nanoparticles with size of 50–200 nm are embedded in carbon aerogels matrix. Fig. 4b shows the magnified TEM image of the red rectangle in Fig. 4a. Amorphous carbon derived from carbon aerogels can be obviously observed outside. Fig. 4c displays the HRTEM image of red rectangle in the Fig. 4b. The Fast Fourier Transform (FFT) and the inverse FFT (IFFT) images of the region identified in Fig. 4c are shown in the inset. The d-spacing value of 0.34 and 0.24 nm are attributed to (110) and (−212) plane of MoO₂. The FFT image can be visualized along the (110) and (−212) planes of the monoclinic MoO₂. The selected area IFFT image show clear lattice fringes of the (110) and (−212) planes with a spacing of 0.34 and 0.24 nm, respectively. Furthermore, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping analysis indicate the homogeneous presence of Mo, O and C elements. In particular, the abundant C further illustrated that the carbon matrix well coated MoO₂ nanoparticles.

Fig. 4 Microstructure characterization of MoO₂@SAC-375 aerogels. (a and b) TEM images, (c) HRTEM image, (the insets are (i) FFT, (ii) IFFT images of the selected area) (d) the HAADF-STEM and EDS elemental mapping images.

The cyclic voltammetry (CV) curves of the MoO₂@SAC-375 electrode at 0.1 mV s⁻¹ over a voltage range of 0.01 and 3.0 V is shown in Fig. 5a. During the first discharge cycle, two distinct reduction peaks at 0.52 and 0.72 V are attributed to the irreversible reduction of the electrolyte and the formation of solid electrolyte interphase (SEI) films.³⁰ These two peaks disappear in the subsequent cycles. The reversible reduction peaks located at 1.26, 1.28 and 1.57 V corresponding to phase transitions of the partially lithiated Li_xMoO₂ are observed.³¹ In the subsequent cycles, these peaks shift slightly to 1.24, 1.25 and 1.52 V, which is attributed to the phase transformations between the monoclinic and orthorhombic phases in the partially lithiated Li_xMoO₂.³¹ The additional peaks below 0.25 V are associated with a three-electron reduction conversion reaction, where Li_xMoO₂ was further reduced to metallic Mo.⁹ The charge and discharge processes of MoO₂@SAC electrode can be divided into two parts over a voltage range of 0.01–3.0 V. Over 1.0 V, Li ions inserted into the structure of MoO₂, resulting in the formation of Li_xMoO₂. When discharged potential is lower than 1.0 V, Li_xMoO₂ would gradually convert to Mo metal and Li₂O, leading to the decrease in intensity of the intercalation and deintercalation peaks with cycles. The Li ions insertion/extraction into/out of MoO₂ can be described as the following eqn (3) and (4).³²

xLi⁺ + xe⁻ + MoO₂ ↔ Li_xMoO₂ (0 ≤ x ≤ 0.98) (3)

Li_0.98MoO₂ + 3.02Li ↔ 2Li₂O + Mo (4)

Fig. 5 (a) CV curves of the MoO₂@SAC-375 electrode at a scan rate of 0.1 mV s⁻¹. (b) Galvanostatic charge–discharge curves of MoO₂@SAC-375 at 1st, 2nd, 5th, 40th, and 100th cycles between 0.01 and 3.0 V (vs. Li⁺/Li) at a current density of 200 mA g⁻¹. (c) Cycling performance and (d) rate capabilities of MoO₂@SAC-X electrodes.

The charge/discharge curves of the MoO₂@SAC-375 electrode at a current density of 200 mA g⁻¹ over a voltage range of 0.01–3.0 V are shown in Fig. 5b. The MoO₂@SAC-375 electrode displays a discharge capacity of 438 mA h g⁻¹ and charge capacity of 318 mA h g⁻¹ with an initial coulombic efficiency (CE) of 72%. The potential plateaus in discharge progress at 1.24 and 1.52 V are consistent with the CV results. More importantly, these plateaus are still visible after the 100th cycle, indicating a good cycle stability. The increase of reversible capacity in initial 40 cycles is typical for MoO₂ materials.^13,33–36 It might be indicated that an activation process related to the partial crystallinity degradation to disorder of the electrodes exists, which is known as electrode activation/formation effect.^8,35

Cycling performance at a current density of 200 mA g⁻¹ for MoO₂@SAC-X electrodes is displayed in Fig. 5c. Obviously, the MoO₂@SAC-375 electrode exhibits the best cycle performance with a reversible capacity of 490 mA h g⁻¹ after 120 deep discharge/charge cycles, much better than those of MoO₂@SAC-350 and MoO₂@SAC-400. It can be seen that MoO₂@SAC (Fig. S5†) and MoO₂@SAC-350 shows the similar electrochemical performance with low capacity and poor cycling stability. This might be related to the lower crystallinity of MoO₂ in the two samples. Compared with MoO₂@SAC-375, MoO₂@SAC-400 exhibits a comparative specific capacity and worse stability. This is because Li ions in MoO₂@SAC-400 are liable to insert MoO₂ structure due to the relative lower carbon content. Therefore, metallic Mo derived from Li_xMoO₂ during the charge process may aggregate to form metal clusters which no longer participate in the conversion reactions, thus leading to the final decay after 100 cycles.⁸ What's more, the cycle performance of the MoO₂@SAC-375 was compared with commercial MoO₂ and pure SAC aerogels. The commercial MoO₂ exhibits much faster capacity fading with a discharge capacity of only 197 mA h g⁻¹ left after 50 cycles at 200 mA g⁻¹ (Fig. S6b†). Under the identical test conditions, the SAC aerogels show a stable but much lower discharge capacity of 187.9 mA h g⁻¹ after 100 cycles (Fig. S6a†). Therefore, the cycle performance results indicate carbon aerogels matrix is beneficial to buffer volume expansion during cycling, while MoO₂ can offer high specific capacity. MoO₂@SAC-375 exhibit the best performance indicate there would be an optimal carbon content in the composite.

Fig. 5d shows the rate capabilities of the MoO₂@SAC-X electrodes. The MoO₂@SAC-375 electrode delivers stable capacity of 574, 483, 425 and 331 mA h g⁻¹ with the current densities increase stepwise from 100 to 200, 500 and 1000 mA g⁻¹, respectively. When the current density finally returns to 100 mA g⁻¹, a reversible capacity of 565 mA h g⁻¹ can be recovered, which is significantly better than the other samples. The excellent rate performance is attributed to the carbon aerogels matrix which enhance the conductivity of the composite.

XRD patterns of the MoO₂@SAC-375 aerogels after 1st and 120th cycles at a current of 200 mA g⁻¹ are shown in Fig. 6. It can be seen that total amorphization of MoO₂ presented after 120 cycles indicating the conversion reactions between MoO₂ and Li. In the cycling process, the MoO₂@SAC-375 aerogels gradually lost their crystallinity and transformed to an amorphous structure, which is beneficial to the improving of the Li ions diffusion kinetics; also, more Li ions could inserted into or be extracted from the anode.

Fig. 6 XRD patterns of the MoO₂@SAC-375 aerogels after 1st and 120th cycles.

Fig. 7 shows the Nyquist profiles Electrochemical Impedance Spectroscopy (EIS) for MoO₂@SAC-350, MoO₂@SAC-375 and MoO₂@SAC-400 electrodes before discharge–charge cycle. In the inset of Fig. 7, the simplified equivalent circuit was used to interpret the measured results.³¹ It can be seen that these plots all consist two parts: a semicircle due to the ohmic resistance and charge transfer resistance at high frequencies, and a short inclined line relating to the ion diffusion in low frequency regions within the anode.³² The semicircle for MoO₂@SAC-350 electrode was much smaller than that of MoO₂@SAC-375 and MoO₂@SAC-400, indicating that the carbon aerogels can facilitate electron transfer from embedded MoO₂ nanoparticles within the whole electrode and thus decrease resistance. Table 1 lists the parameters of the equivalent circuit for MoO₂@SAC-350, MoO₂@SAC-375 and MoO₂@SAC-400 after fitting the diameter of the semi-circular curve. Obviously, R_ct of MoO₂@SAC-375 electrode was much smaller than that of MoO₂@SAC-350 and MoO₂@SAC-400, suggesting that the electron transference of the former is prior to that of the latter.

Fig. 7 Electrochemical impedance spectroscopy (EIS) for MoO₂@SAC-X electrodes. Inset: the equivalent circuit used to fit the experimental data and the plot enlarged over the high frequency range.

Table 1 Impedance parameters calculated from an equivalent circuit model

Samples	Rs	Rct
MoO2@SAC-350	2.591	236.5
MoO2@SAC-375	1.721	112.6
MoO2@SAC-400	1.534	357.3

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Table S2† shows the comparison between the MoO₂/carbon anodes prepared by other typical methods and the present method. It can be seen that although the capacity is not superior than previous materials, the MoO₂/carbon anode prepared by our present method shows a relatively longer cycle number of 120 cycles. We believed that cycle stability of Li-ion anodes is as important as the capacity. In addition, as a most important innovation, we proposed a new method to chelate alginate with anionic group and the content of chelator is controllable during the synthesis procedure.

On the whole, the excellent electrochemical performance of MoO₂@SAC aerogels can be attributed to the high specific capacity of MoO₂ and the unique network carbon matrix feature. The size of MoO₂ nanoparticles in the composite is 50–200 nm, which is beneficial to increase the contact area of active materials with electrolyte and shorten the Li ions diffusion length significantly. The porous structure of carbon aerogels matrix can decrease the electrochemical impedance of the electrode and enable fast charge flow, and can be able to accommodate the volume expansion of MoO₂ nanoparticles during charge/discharge cycles, which ensuring the high cycling stability. It is indicated that the synergistic effect between MoO₂ and carbon aerogels matrix lead to the brilliant Li-storage property.

Experimental

Materials

Sodium alginate ((C₆H₇O₆Na)_n) was purchased from aladdin. Hydrochloric acid and ammonium molybdate tetrahydrate (AMM, (NH₄)₆Mo₇O₂₄·4H₂O) were purchased from Sinopharm Chemical Reagent (Shanghai, China). All the reagent were used without further purification.

Synthesis of MoO₂@SAC aerogels

First, 1.08 g AMM and 1.5 g sodium alginate were dissolved in 100 mL H₂O at room temperature under magnetic stirring to form a homogeneous solution. Keep the above solution stand overnight for de-aeration. Then, the solution was added into 0.2 M hydrochloric acid using a syringe needle drop by drop to form hydrogels. The obtained hydrogels were washed with ultra-pure water for three times, then frozen at −58 °C and lyophilized. The lyophilized xerogels were stabilized in air at 350, 375 and 400 °C for 1 h (ramp rate: 5 °C min⁻¹). Finally, the stabilized samples were carbonized and reduced in a tube furnace at 600 °C under N₂ atmosphere for 1 h to obtain MoO₂@SAC aerogels (ramp rate: 4 °C min⁻¹). The products were denoted as MoO₂@SAC-X (X = 350, 375, 400, according to the different stabilization temperature). In order to investigate the effect of stabilization, the sample named MoO₂@SAC was synthesized by directly annealing the lyophilized xerogels at 600 °C under N₂ atmosphere without stabilization in air. SAC aerogels were prepared by pyrolysis of alginic acid aerogels without adding AMM at 600 °C under N₂ atmosphere.

Characterization

The phase structures were characterized by X-ray diffraction (XRD, DX2700, China) operating with Cu Kα radiation (l = 1.5418 Å) at a scan rate (2θ) of 3° min⁻¹ with an accelerating voltage of 40 kV and an applied current of 30 mA, ranging from 5 to 90°. Thermogravimetric analysis (TGA) measurement was carried out on an EXSTAR TG 6300 instrument (Seiko Instruments, Japan) with a heating rate of 10 °C min⁻¹ in air. Raman spectrometer (Renishaw, inVia) was used to acquire the Raman spectra with a 514.5 nm laser at room temperature. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method from the data in a relative pressure (P/P₀) range between 0.05 and 0.20; pore size distribution plots were derived from the adsorption branch of the isotherms based on the BJH model. The morphology and microstructure of the samples were investigated by field emission scanning electron microscopy (FESEM; JSM-7001F, JEOL, Tokyo, Japan), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a FEI Tecnai 20 TEM with an accelerating voltage of 200 kV with energy dispersive X-ray spectroscopy (EDS).

Electrochemical measurements

The MoO₂@SAC aerogels were mixed with acetylene black and poly(vinylidene fluoride) (PVDF) at a weight ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidone (NMP) solvent to form a slurry. Then, the resultant slurry was uniformly pasted on a Cu foil substrate. The prepared electrode sheet was dried in a vacuum oven at 120 °C for 10 h and then pressed. The active material loading density of the electrode is 1.9 mg cm⁻². The electrolyte consisted of a solution of 1 M LiPF₆ in ethylene carbonate (EC)–dimethyl carbonate (DMC)–diethyl carbonate (DEC) (1 : 1 : 1, in wt%). CR2016-type coin cells were assembled in a glove box for electrochemical characterization. Lithium metal foil was used as the counter and reference electrode. The discharge and charge measurements were conducted using a cell testing instrument (LAND CT2001A) over the potential range of 0.01 to 3.0 V. Cyclic voltammetry (CV) (0.01 to 3.0 V, 0.1 mV s⁻¹) was performed using a CHI 760E electrochemical workstation. Electrochemical impedance spectroscopy (EIS) spectra were measured over a frequency range of 0.01 Hz to 100 kHz.

Conclusions

In conclusion, we synthesized a unique MoO₂/SAC aerogels by a one step in situ method using an environmental friendly alginate as the carbon precursor. In the composite, MoO₂ nanoparticles with a diameter 50–200 nm are embedded in carbon aerogels matrix to form a network structure. When used as anodes for LIBs, the composite stabilized at 375 °C exhibited a high specific capacity of 574 mA h g⁻¹ and good cycle performance (i.e., the discharge capacities reach 490 mA h g⁻¹ cycled at 200 mA g⁻¹ for 120 cycles). The high electrochemical performance is attributed to their unique hierarchical porous structure where the synergistic effect of MoO₂ nanoparticles and the carbon aerogels matrix greatly enhances the reversible capacity, cycling stability and rate performance. Moreover, alginate acted as the carbon precursor and template, is a common biopolymer derived from biomass, which makes the facile synthesis process a green and environmentally friendly strategy.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (no. 51473081 and 51503109), Research award fund for outstanding young scientists in Shandong province (grant no. BS2014CL006), Qingdao Applied Basic Research Project (16-5-1-85-jch), and the start-up funding from Peking University and Young Thousand Talented Program.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns, N₂ sorption data, electrochemical performance and SEM images of the controlled experiments. See DOI: 10.1039/c6ra22642f

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