One-Step Synthesis of Novel Mesoporous Three-Dimensional GeO2 and Its Lithium Storage Properties
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. The reaction of GeO2 with Li involves two steps: 1) GeO2 + 4Li à Ge + 2Li2O; 2) xLi + Ge ⇌ LixGe (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. 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.
 Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P.A. van Aken, J. Maier, Adv. Mater., 22 (2010) 2247-2250.
 H.P. Jia, P.F. Gao, J. Yang, J.L. Wang, Y.N. Nuli, Z. Yang, Adv. Energy Mater., 1 (2011) 1036-1039.
 M. Winter, J.O. Besenhard, J.H. Albering, J. Yang, M. Wachtler, Prog. Batteries Battery Mater. 1998, 17, 208−214.
 K.H. Seng, M.H. Park, Z.P. Guo, H.K. Liu, J. Cho, Angewandte Chemie (International ed. in English), 51 (2012) 5657-5661.
 M.H. Park, Y. Cho, K. Kim, J. Kim, M. Liu, J. Cho, Angew. Chem.-Int. Edit., 50 (2011) 9647-9650.
 C.K. Chan, X.F. Zhang, Y. Cui, Nano Lett., 8 (2008) 307-309.
 L.C. Yang, Q.S. Gao, L. Li, Y. Tang, Y.P. Wu, Electrochem. Commun., 12 (2010) 418-421.
 Y. Kim, H. Hwang, K. Lawler, S.W. Martin, J. Cho, Electrochimica Acta, 53 (2008) 5058-5064.
 J.S. Chen, C.M. Li, W.W. Zhou, Q.Y. Yan, L.A. Archer, X.W. Lou, Nanoscale, 1 (2009) 280-285.