One-Step Synthesis of Novel Mesoporous Three-Dimensional GeO2 and Its Lithium Storage Properties

In order to meet the requirements of high energy density as well as high power performance, a worldwide effort has been made to develop novel high-performance electrode materials to keep pace with the fast increasing market demands. Various compounds that form intermetallic phases (so-called “alloys”) with lithium have been proposed as alternative anode materials, such as silicon (Si) [1-2], tin (Sn) [3] and germanium (Ge) [4]. Compared to Si, less attention has been paid so far on Ge due to its higher costs. Nevertheless, its good lithium diffusivity (400 times higher than in silicon) and high electronic conductivity (ca. 104 times higher than silicon) enable it as a promising anode material for the next-generation LIBs. Currently, researchers report on different structures of Ge-based materials, such as nanotubes [5], nanowires [6], porous structures [7] and Ge-based composite materials [4].

So far, only a few studies focus on GeO₂ as anode material. GeO₂ is of interest for application as negative electrode material due to its high theoretical reversible capacity (1125 mAh g^-1 based on 4.25 mol Li per mol Ge), low operation voltage (0.7 V to 0 V vs. Li/Li⁺) and higher thermal stability compared to Ge.[8] The reaction of GeO₂ with Li involves two steps: 1) GeO₂ + 4Li à Ge + 2Li₂O; 2) xLi + Ge ⇌ Li_xGe (x ≤ 4.25). While the first reaction can be considered as to a large part electrochemically irreversible, resulting in a large irreversible charge loss by formation of Li₂O, the lithiation/de-lithiation reaction in step 2) can be regarded as reversible, providing the discharge capacity of GeO₂. The high irreversible capacity during the first lithiation process is also well-known for SnO₂-based anodes and is one of the main reasons hindering so far the application in commercial lithium-ion batteries.[9] In general, the synthesis of GeO₂ can be achieved by different methods, including hydrolysis reactions from a Ge precursor, chemical vapor deposition (CVD) and sputter processes.

In this work, novel mesoporous three-dimensional GeO₂ was successfully synthesized by a facile one-step synthesis method followed by mixing with graphene using a spray drying process. The as-prepared GeO₂ shows a bean-like morphology with a particle size of 400 to 500 nm in length and 200 to 300 nm in diameter (Figure 1a-d). In a further step, we take the advantages of graphene (low-dimensional carbon material with high electronic conductivity; excellent mechanical properties; graphene can contribute to the capacity) to further enhance the electrochemical performance of b-GeO₂. As illustrated in Figure 2, it can be clearly observed that the GeO₂ particles are tightly anchored on or wrapped within the graphene sheets. Notably, during the mixing process, GeO₂ particles are still homogenously dispersed within or firmly encapsulated by the graphene sheets. The b-GeO₂ without any additional conductive surface layer shows a high reversible capacity for lithium storage of 845 mAh g^‑1 after 100 cycles, with nearly no capacity fading. When graphene was employed to be mixed with GeO₂ via a spray drying method, the electrochemical performance is further significantly improved. The b‑GeO₂/graphene composite electrode gives a higher de-lithiation capacity of 1,021 mAh g^-1 for 200 cycles at 0.2C and a high rate capability of up to 5C. This superior electrochemical performance benefits from its uniformly dispersed mesoporous structure, in which the GeO₂ particles are homogeneously distributed on or within the conductive graphene sheets.

Figure 1. Low-resolution (a, b) and high-resolution-resolution (c, d) SEM images of b GeO₂.

Figure 2. (a, b) SEM images of the b-GeO₂/graphene composite prepared by spray drying method.

References

 

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