Flexible dimensional control of high-capacity Li-ion-battery anodes: from 0D hollow to 3D porous germanium nanoparticle assemblies.

Adv. Mater. 2010, 22, 415–418 2010 WILEY-VCH Verlag Gm Porous nanostructured metals have been investigated for their various applications, such as catalysts, gas and thermal sensors, and electronics. Approaches for preparing such porous materials include coating templates and removing these templates by etching or thermal annealing. Recently, the deposition of metals on porous biomineral templates (diatoms with porous silica frustules with a 3D structure) to acquire 3D porous metals has been reported. Another approach to 3D porous metal or metal oxides has been widely used in photonic devices, and this method involves the infiltration of a silica template within the void spaces of a face-centered cubic colloidal crystal mode of microspheres (opals). Using this method, Ge has been synthesized through a two-step process consisting of hydrolysis of an alkoxide and subsequent reduction to metallic Ge. On the other hand, CVD or electrodeposition using Ge2H6 [15] or GeCl4 [16] above 300 8C has also been used, but GeO2 formation from the reaction between Ge and SiO2 was inevitable. Oxides resulted in the formation of a lithium inactive Li2O phase during the first charge (lithium insertion), leading to a decrease in coulombic efficiency. Furthermore, the preparation of the silica opal template was very complex and laborious. Recently, Cho and co-workers reported a simple synthetic method for preparing 3D porous Si particles, whereby the physical blending of a SiO2 template and a Si precursor leads to a 3D porous bulk Si particle with a pore wall thickness and a particle size of >40 nm and >10mm, respectively. Currently, many projects focus on alternative high-capacity anode materials for the replacement of currently used graphite electrodes in Li secondary batteries. Such candidates are Si, Sn, and Ge. The latter metal has also been in the spotlight because of its high theoretical capacity (ca. 1600mAhg ) and faster Li diffusivity than Si (400 times faster). However, mechanical stresses, related to the volume changes during lithium alloying and leaching (Mþ xLiþþ xe $ LixM (0 x 4.4)), induce a rapid decay in mechanical stability. As a consequence, the electrode suffers from cracking and crumbling (pulverization) as well as from a consequent loss of electronic interparticle contact and exfoliation from the current collector. In this regard, porous nanoparticles with ordered pore arrays are the best candidates to minimize and accommodate the volume changes of the metals during lithium alloying and leaching. To date, there have been no reports on the synthesis of 0D and 3D porous Ge nanoparticle assemblies fabricated by controlling the amount of template and Ge precursors used. Herein, we report the synthesis and electrochemical properties of 0D hollow and 3D porous Ge nanoparticle assemblies prepared by etching a thermally annealed physical mixture of SiO2 and ethyl-capped Ge gels at 800 8C. By tuning the weight fraction of ethyl-Ge gels relative to the amount of SiO2, either 0D hollow nanoparticles or 3D porous Ge nanoparticle assemblies (Scheme 1) can be obtained. The 3D porous Ge retains almost 99% of its capacity after 100 cycles at 1C rate (1⁄4 1.6Ag ). Figure 1a shows the SEM image of the SiO2 templates, which consists of relatively monodispersed nanospheres with a diameter of about 200 nm. Using a weight fraction of SiO2 and the ethyl-capped Ge gel solution of 7:3 (SiO2/Ge), hollow Ge particles with a size of around 220 nm were obtained and no ordering between the nanoparticles was observed (Fig. 1b). Upon increasing the ratio to 5:5, a similar particle morphology to the 7:3 ratio (SiO2/Ge) was observed, but the regularity between the particles appeared to increase (see Supporting Information, Fig. S1). When the weight ratio was increased to 3:7 (SiO2/Ge), relatively well-ordered 3D porous nanoparticle assemblies with a pore size of around 200 nm and a wall thickness of 20 nm were observed (Fig. 1c and d). The difference between the 0D hollow and 3D porous Ge assemblies is their regularity as shown in Figure 1. 0D hollow Ge nanoparticles could be dispersed into a solvent, whereas 3D porous Ge nanoparticles maintained their form when immersed into a solvent. In contrast to previously reported 3D porous Si bulk particles with a similar weight fraction, the pore wall thickness of our assemblies is half the size and the presence of ordered porous nanoparticles can be observed. This difference can be associated to the treatment in vacuum for 10min to introduce a homogenous distribution of the gel solution between the particles. The X-ray diffraction (XRD) pattern of both the 0D and 3D Ge nanoparticle assemblies confirmed the presence of the cubic diamond phase (Supporting Information, Fig. S2).