Hydrothermal synthesis of hollow Ca₂Ge₇O₁₆ microspheres as high-capacity anodes for Li-ion batteries with long cycling life

Abstract

Urchin-like Ca₂Ge₇O₁₆ hollow spheres have been successfully synthesised by hydrothermal methods assisted with urea or N-methyl urea. When tested as anode materials, the as-obtained urchin-like hollow spheres consisting of nanorods with diameters of about 20 nm could deliver a capacity of 603 mA h g⁻¹ after 200 cycles at a current density of 1000 mA g⁻¹.

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The increase in energy crisis awareness has driven people to pursue advanced energy storage and conservation technologies.^1–5 As one of the most promising technologies for electric vehicles and hybrid electric vehicles (EVs and HEVs), lithium-ion batteries (LIBs) have attracted great attention in the design of alternative electrode materials with commercial graphite.^6–13 To date, significant progress has been made in the exploration of metal oxides, sulphides, silicon (Si), tin (Sn), and germanium (Ge).^14–22 Among them, germanium-based materials have received increasing attention due to their excellent characteristics such as effective cost, high theoretical specific capacity, environmental benignity, and relatively low working potential.^22–26 However, pure Ge anodes suffer from short cycle life due to their structural pulverisation during lithium alloying/de-alloying processes.^23,24

To improve the electrochemical performance in cycle life and rate characteristics, techniques involving Ge nanoparticles,²⁷ Ge/carbon nanocomposites,^28–30 binary germanium compounds and ternary germanium materials^31–36 have been investigated. Among these materials, Ca₂Ge₇O₁₆ has been shown to improve the performance of germanium-based materials and has obvious advantages over other systems due to an extra material formed during cycling in situ that further buffers the volume change.^34–36 Hollow structures are considered as an effective structure for the alleviation of mechanical stress resulting from lithium alloying/de-alloying processes.^37–39 Combining the above-mentioned factors, we synthesised urchin-like Ca₂Ge₇O₁₆ hollow spheres with outstanding electrochemical Li storage properties via a simple hydrothermal route. Moreover, the as-synthesised Ca₂Ge₇O₁₆ spheres could deliver a capacity of 603 mA h g⁻¹ after 200 cycles at a current density of 1000 mA g⁻¹.

The urchin-like Ca₂Ge₇O₁₆ hollow spheres were synthesised in a hydrothermal system with germanium dioxide, calcium chloride and N-methyl urea. Interestingly, the substructures of the urchin-like Ca₂Ge₇O₁₆ hollow spheres could be changed by adjusting the amount of N-methyl urea. When the amount of N-methyl urea was 14 mmol, the urchin-like hollow spheres (UHS-1) were obtained. Fig. 1a–c show the SEM images of UHS-1 at different magnifications. Fig. 1a indicates that the sample is composed of the well-dispersed urchin-like spheres with diameters ranging from 3 to 5 μm. Moreover, it can be clearly seen that the broken spheres are hollow. In the magnified SEM images (Fig. 1b and c), one can find that the urchin-like spheres are composed of nanowire bundles, and the diameters of the corresponding nanowires are about 25 nm. When the amount of N-dimethyl urea was 3 mmol, urchin-like hollow spheres with diameters ranging from 4 to 8 μm (UHS-2, Fig. 1d and e) were obtained. The magnified SEM image in Fig. 1f indicates that the diameters of the nanowires on UHS-2 are about 50 nm. Fig. 2a and b show the XRD patterns of the as-synthesized UHS-1 and UHS-2, respectively. Both of the XRD peaks can be well indexed to the orthorhombic phase of Ca₂Ge₇O₁₆ (JCPDS Card no. 34-0286).

Fig. 1 (a–c) SEM images of UHS-1 and (d–f) UHS-2.

Fig. 2 XRD patterns of (a) UHS-1 and (b) UHS-2.

To further investigate the substructures of the urchin-like Ca₂Ge₇O₁₆ hollow spheres, transmission electron microscopy (TEM) characterisations were performed. Fig. 3a shows a very small hollow structure that was not observed in the SEM images. Fig. 3b shows some nanowires that are apart from the hollow structures. The high-resolution TEM images in Fig. 3c and S1† demonstrate the poor crystallinity of UHS-1, which may be attributed to the residual organics on the surface and will be discussed in the following section. In addition, we found that UHS-2 could not be effectively characterised by TEM due to its large size. The TEM image in Fig. 3d shows UHS-2 with an urchin-like structure. Because of its large size, we cannot clearly see the hollow structure from the TEM image. However, the SEM images in Fig. 1d, e and S2† demonstrate the existence of the hollow spheres.

Fig. 3 (a–c) TEM images of UHS-1 and (d) UHS-2.

To investigate the morphological evolution and growth mechanism of the urchin-like Ca₂Ge₇O₁₆ hollow spheres, time-dependent morphological evolution was carried out. Fig. S3† shows a series of SEM images of intermediate products reacted at different times (10 min, 20 min, and 30 min) at 240 °C. At the beginning, many nanoparticles were present, as shown in Fig. S3a.† When the time increased to 20 min, several nanowire bundles formed, as shown in Fig. S3b.† The sphere-like structure (Fig. S3c†) was generated after a reaction time of 30 min. During the reaction process, N-methyl urea not only offered an alkaline environment, but also hydrolysed in the aqueous solution to produce bubbles of NH₃, CO₂ and CH₃NH₂. These auto-generated CO₂, NH₃ and CH₃NH₂ bubbles served as self-templates to facilitate the formation of the hollow structures. During the Ostwald ripening process, the urchin-like hollow spheres appeared, governed by the crystal growth habit of Ca₂Ge₇O₁₆. Based on as-presented results, we attributed the formation of the urchin-like Ca₂Ge₇O₁₆ hollow spheres to three factors: (i) the growth habit of the orthorhombic Ca₂Ge₇O₁₆ crystals, which facilitates their one-dimensional growth;³⁷ (ii) the auto-generation of CO₂, NH₃ and CH₃NH₂ bubbles as self-templates, which helps the formation of hollow structures;^40,41 and (iii) the lower energy of the spherical structure, which is stable due to its lower free energy.

In order to study the influence of other synthetic parameters on the structure of Ca₂Ge₇O₁₆, we also carried out some controlled experiments. When the temperature was lowered to 180 °C while keeping other conditions constant, the hollow structure could be still obtained, as shown in Fig. S4a.† This indicates that we could easily prepare the urchin-like Ca₂Ge₇O₁₆ hollow spheres at a wide range of temperatures (180–240 °C). When N-methyl urea was not used and other conditions were kept the same, non-uniform nanowires were obtained (Fig. S4b†). This demonstrates that N-methyl urea is extremely important for modulating the growth of the urchin-like Ca₂Ge₇O₁₆ hollow spheres. When N-methyl urea was replaced by the same concentration of ammonium hydrogen carbonate, some nanowires together with some microparticles were produced, as shown in Fig. S4c.† The ammonium hydrogen carbonate was dissociated into positive ammonium ions and negative hydrogen carbonate ions, which coexist with NH₃ and CO₂ due to the reversible reaction between them. Although NH₃ and CO₂ existed in the reaction system with ammonium hydrogen carbonate, obviously different results were produced. In addition, we found that urea could play the same role as N-methyl urea in our reaction system. The sample synthesised with urea is shown in Fig. S5;† this experiment supported the conclusion about the effects of N-methyl urea.

Due to their unique structures, an excellent electrochemical performance is expected for the urchin-like Ca₂Ge₇O₁₆ hollow spheres. The electrochemical performance of two urchin-like Ca₂Ge₇O₁₆ hollow spheres was tested using a galvanostatic discharge–charge technique. The discharge–charge voltage profiles of different cycles under a current density of 1 A g⁻¹ over a voltage window from 0 to 3.0 V vs. Li ⁺/Li are shown in Fig. 4a and b. The first discharge capacities of UHS-1 and UHS-2 are 1277 and 1262 mA h g⁻¹, respectively; however, the first charge capacities of the two samples are 255 and 263 mA h g⁻¹, and their corresponding columbic efficiencies only reach 20% and 21%, respectively. Based on previous reports of binary and ternary germanium compounds,^31–36 the lithium storage mechanism of Ca₂Ge₇O₁₆ can be described by the following eqn (1)–(3):

Ca₂Ge₇O₁₆ + 28e⁻ + 28Li⁺ → 2CaO + 7Ge + 14Li₂O (1)

7Ge + 30.8e⁻ + 30.8Li⁺ ↔ 7Li_4.4Ge (2)

Ge + 2Li₂O ↔ GeO₂ + 4Li⁺ + 4e⁻ (3)

Fig. 4 The discharge–charge voltage profiles of different cycles of (a) UHS-1 and (b) UHS-2 at a current density of 1 A g⁻¹; (c) the cycle performances of UHS-1 and UHS-2 at a current density of 1 A g⁻¹; and (d) the rate performances of UHS-1 and UHS-2 electrodes at different current densities.

Eqn (1) is an irreversible chemical reaction, and the generation of GeO₂ is partially reversible, which contributed to the low coulombic efficiencies in the first few cycles. After several cycles, the coulombic efficiencies of the two samples exceeded 99% and maintained stability.

The cycling performances of UHS-1 and UHS-2 are shown in Fig. 4c. It is clearly found that the capacities of both UHS-1 and UHS-2 faded during the first ten cycles. After that, the capacities of the two samples increased separately to about 590 and 360 mA h g⁻¹ at the 100th and 40th cycle. To investigate the reason for the capacity decline and rise process, we took some further measures. UHS-1 and UHS-2 samples were calcined in air at 400 °C (heating rate of 3 °C min⁻¹) for 2 h. The cycling performance of the two samples after calcining are shown in Fig. S6.† The time of the capacity decay along with the time of recovery shortened. The capacities reached the maxima at about 20 cycles. Thermogravimetric (TG) analysis (Fig. S7†) indicated a gradual weight loss as the temperature increased to 700 °C, which can be attributed to the loss of organic molecules within UHS-1. Residual organics may cause the dramatic decline in capacity and hinder its recovery. The increasing capacity of UHS-1 during cycling suggests an electrochemical activation process, similar to the phenomenon reported in the literature.³⁶ In addition, the capacity of UHS-1 was much higher than the capacity of UHS-2, possibly due to the influence of the size. The small size could make the area of sample in contact with the electrolyte larger; thus, more lithium ions would participate in lithiation/delithiation due to the shorter lithium ion diffusion distance. SEM images of the Ca₂Ge₇O₁₆ anodes of UHS-1 and UHS-2 before cycling (Fig. S8a and b†) and after 200 cycles (Fig. S8c and d†) were collected. Before cycling, some broken hollow spheres and many nanowire bundles broken from hollow spheres were observed. After 200 cycles, only thick nanowires are seen. After the capacity reached an almost stable level at 500 mA g⁻¹, the rate capabilities of UHS-1 and UHS-2 were measured (Fig. 4d). A good capacity retention was obtained at each current ranging from 500 mA g⁻¹ to 4 A g⁻¹. When cycling at high current densities of 4 A g⁻¹, the capacities of UHS-1 and UHS-2 were maintained at 420 and 180 mA h g⁻¹, respectively. In summary, the excellent properties of UHS-1 make it deserving of study for potential application in lithium-ion batteries.

In conclusion, we have successfully developed a simple hydrothermal method for the preparation of urchin-like Ca₂Ge₇O₁₆ hollow spheres. The subunits of the as-obtained urchin-like hollow spheres could be changed by adjusting some synthetic parameters. Moreover, the formation mechanism of the urchin-like Ca₂Ge₇O₁₆ hollow spheres was discussed. Finally, the as-obtained urchin-like hollow spheres displayed a high discharge capacity of 603 mA h g⁻¹ after 200 cycles at a current density of 1000 mA g⁻¹.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51302079) and the Young Teachers' Growth Plan of Hunan University (Grant no. 2012-118).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08927h

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