Germanium Nanocrystals-MWCNTs Composites as Anode Materials for Lithium Ion Batteries

Germanium Nanocrystals-MWCNTs composites as anode materials for Lithium Ion Batteries

S. Goripartia,b, E. Mielea, A. Scarpellinia, S. Marrasa, M. Pratoa, A. Ansaldoa, F. De Angelisa, L. Mannaa , R. Proietti Zaccariaa and C. Capigliaa

.

a Istituto Italiano di Tecnologia, Via Morego 30, Genova I-16163, Italy b University of Genova, Genova 16145, Italy

Germanium (Ge) nanocrystals with and without multiwalled carbon nanotubes (MWCNTs) are synthesised in via solvothermal approach. The active anode material is characterized by X-ray Diffraction pattern (XRD), Transmission Electron Microscopy (TEM) and Raman spectroscopy. Cyclic voltammetry and galvanostatic charging and discharging are employed to characterize electrochemical behaviour. The results show Ge-MWCNTs nanocomposite improved cycling performance with higher capacity retention compared to pure Ge electrode: discharge capacity of ~ 1170 mAh g-1 after 10 cycles and good capacity retention are achieved.

Introduction

The continuous increasing in the energy demand from most of the electronic devices, spanning from portable electronics to electric vehicles, has been pushing for the development of new highly efficient materials for lithium ion batteries (LIBs), especially capable of providing high energy and high power density [1-6]. Among the known active anode materials, the group IV-A has gained special attention owing to some of its elements such as silicon, germanium and tin, showing a specific capacity of 4200 mAh g-1, 1600 mAh g-1 and 990 mAh g-1, respectively. Clearly, these values represent very good results especially when compared to the commercial graphite anodes having a specific capacity of 372 mAh g-1 [4-7]. However, the group IV-A based anode materials suffer huge capacity fading during charging-discharging process due to high volume changes, thus hindering their use in market ready-LIBs. Several efforts try to overcome the volume change issue, the most common approach being the reduction to the nanoscale of the particles forming the anode material. In particular, various morphological nanostructures like nanowires, nanotubes, nanoporous and nano/micro porous structures have been proposed [4-9]. The effect of the particles size reduction is the release of stress caused by the volume expansion. Although nanostructured materials can tolerate a fair amount of mechanical stress and exhibit improved performance with respect to bulky materials, they usually suffer from capacity retentions. Numerous studies have been proposed to address this problem, especially oriented towards the improvements of the electrochemical performance of the nano-sized forms of metals such as copper, tin and nickel, either carbon coated or alloyed with carbonaceous and non-carbonaceous materials [10-14]. Recent studies have showed that these active nanostructured materials, when combined with composites of conductive matrices such as graphene, carbon nanofibers and carbon nanotubes, exhibit improved electrode performance [13-20]. In addition, these composites can modify the surface of the active nano-sized materials, therefore reducing their aggregation during the lithium alloy/de-alloy reactions. However, further developments towards active nanostructures with conductive matrices are necessary to improve the capacity retention and achieve longer cycling stability. Among IV-A materials, germanium shows very interesting properties, such as high diffusion of lithium and high electrical conductivity due to narrow band gap (0.6 eV). These features make

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germanium an attractive anode material especially at high current rates [14, 21, 22]. Furthermore, the theoretical specific capacity of Ge exceeds four times the present graphite anode capacity, leading to anodes with improved lithium storage capability and cycle life [9, 21-25]. Recently, Yuan et. al reported Ge/reduced graphene with excellent performance and high capacity retention (~ 1000 mAh g-1 after 600 charge-discharge cycles at a current rate of 0.2C) [15]. Moreover, germanium nanowires obtained from vapour-liquid-solid mechanism, exhibited good capacities of 800 and 600 mAh g-1 respectively at the current rates of 1C and 2C [26], and their stability at very high current rates (60 and 100C) has also been proved. The conventional methods to prepare Ge nanostructures involve techniques like physical and chemical vapour deposition, gas phase photolysis, laser ablation, molecular beam epitaxy and electron beam evaporation [13, 27-28]. The synthesis of Ge by solution methods requires high temperature conditions and/or the employment of hazardous materials as reducing agents, such as LiAlH 4, Butyl lithium [29,30]. Here we report about an easy approach to produce 5-10 nm Ge nanocrystals with multiwalled carbon nanotubes (MWCNTs), based on the solvothermal method. We show that these composites have improved cycle performances in particular Ge-MWCNTs exhibit a capacity nearly 1170 mAh g-1 after 10 cycles at a current rate of 100 mA g-1, making them suitable as anode materials for application in high performance LIBs.

Experimental Section

Germanium nanocrystals and their composites with MWCNTs were prepared by solvothermal method. Ge nanocrystals and Ge-MWCNTs were characterized using a JEOL JEM 1011 transmission electron microscope (TEM) operating at 100 kV accelerating voltage. X-ray diffraction pattern from powder was recorded with Rigaku SmartLab 9 kW diffractometer with an X-ray source from Cu Kα with 40 kV and 150 mA. Raman spectroscopy measurements were carried out with Renishaw inVia Micro Raman equipped with laser source at 532 nm (13-mW power) through a 50× objective (LEICA N PLAN EPI 50/0.75).

Figure 1. X-ray diffraction pattern of Germanium nanocrystals.

The electrode was prepared by coating on a copper foil a slurry of either powder of pure Ge nanocrystals or Ge-MWCNTs mixed with super P carbon and polyvinyledenefluoride in the ratio

ECS Transactions, 62 (1) 19-24 (2014)



of 80:10:10. The coin cell types 2032 were assembled in an argon filled glovebox (MBraud Unilab) with oxygen and water levels kept below 2 ppm. For the half cell fabrication Ge-based electrode material were used versus lithium metal and celgard-2400 dipped in 1M LiPF6 in EC: DMC (1:1 V%) as a separator. The cells were tested with cyclic voltammetry and galvanostatic charge-discharge by using a multichannel battery tester (Biologic MPG-2) in the range from 0.005 V to 1.5 V versus Li/Li+.

Figure 2. Transmission electron microscopy images of a, b) Ge nanocrystals; c) Ge-MWCNTs; d) Raman spectrum of Ge-MWCNTs.

Results and Discussions Ge nanocrystals synthesis was carried out with solvothermal approach by reduction of GeI2. The X-ray diffraction (XRD) pattern of the synthesised Ge nanocrystals is shown in Figure 1. Three broad diffraction peaks with 2Ө values at 27.15º, 45.30º, 53.65º can be observed. They well represent the Ge cubic crystal structure, in excellent agreement with the standard value of Ge lattice constant of a = 5.6512 Å (JCPDS card No. 03-065-0333). The broadening of the diffraction peaks are related to the small size of Ge nanocrystals. Figures 2a and 2b show TEM images of the synthesised Ge nanocrystals. In particular, we can observe an average size between 5 nm and 8 nm. Figure 2c illustrate the situation where Ge nanocrystals are densely and uniformly distributed on the surface of a MWCNT. The presence of Ge and MWCNTs was confirmed by Raman measurements, as shown by the spectrum in Figure 2d. The vibrational peaks are observed at 298 cm-1, associated to Ge, and at 1344 cm-1 and 1579 cm-1, both associated to the presence of MWCNTs. In particular, the 1344 cm-1 peak describes the D band, which corresponds to defects and disorder of hexagonal




graphitic layers, while the 1579 cm-1 peak is associated to the G band, which is due to vibration of sp2 bonded carbons in 2-dimentional hexagonal lattices. The electrochemical performance of Ge nanocrystals and Ge-MWCNTs composites was evaluated by cyclic voltammetry and galvanostatic charge-discharge cycles in CR2032. Figure 3a shows the voltammograms of Ge (black line) and Ge-MWCNTs (red line). The latter shows a lithiation reaction in three steps, at ~ 0.53 V, 0.26 V and 0.12 V vs. Li/Li+. The peaks in the voltammograms profile are in good agreement with literature describing lithiation and alloying process of Ge (Li 9Ge4 and Li7Ge4 followed by Li15Ge4 and Li22Ge4) [24]. Likewise, the de-lithiation also occurred in three steps, which are at 0.52 V, 0.60 V and 0.74 V vs. Li/Li+. The observation of multiple peaks in the cyclic voltammograms is indicative of lithium insertion into equipotential sites and the presence of multiple sharp peaks suggest that various Li-Ge phases are forming during the electrochemical lithiation and de-lithiation. In addition to lithium storage capacity of Ge, the MWCNTs contribution was observed, which was confirmed from de-lithiation peak at ~ 0.2 V vs. Li/Li +.

Figure 3. a) The first cycle of cyclic voltammograms of Ge nanocrystals (black) and Ge-MWCNTs (red) at the scan rate of 0.02 mV s-1 (potential between 1.5 V and 0.005 V); Galvanostatic charging-discharging curves of b) Ge nanocrystals and c) Ge-MWCNTs at current of 100 mA g-1 (all the potential vs. Li/Li+ are presented here).

Galvanostatic charge-discharge cycles of Ge nanocrystals and Ge-MWCNTs were carried out in the potential range of 1.5-0.005 V vs. Li/Li+ with the results shown in Figure 3. In particular, Figure 3b




and Figure 3c show the first, second and 10th charge-discharge curves at the current rate of 100 mAh g-1 for Ge and Ge-MWCNTs, respectively. The Ge-MWCNTs exhibited better cycling performance with higher specific capacity. For Ge-MWCNTs, the first cycle discharge and charge capacities are 1795 mAh g-1 and 1318 mAh g-1, respectively, with a columbic efficiency of 73%. The large irreversible capacity can be ascribed to the formation of solid electrolyte interface (SEI) at potential between 1.0 V and 0.7 V vs. Li/Li+. As expected, the irreversible capacity is reduced during cycling: the obtained discharge and charge capacities after 10th cycle were 1339 mAh g-1 and 1167 mAh g-1 with a columbic efficiency of 88%. Whereas electrodes formed by Ge nanocrystals presented a first discharge and charge capacities of 1650 mAh g-1 and 1021 mAh g-1 with a columbic efficiency of 62%. At the 10th cycle, the charge capacity was limited to ~ 680 mAh g-1. The poor cycling stability (Figure 3b) of Ge nanocrystals electrode is related to both the Ge nanoparticles aggregation and to the insufficient conductive channel during the lithiation and de-lithiation process. The Ge-MWCNTs composite electrode showed better performance owing to the uniform distribution of Ge nanocrystals on the surface of MWCNTs, which avoids particles agglomeration (as shown Fig. 2c) and facilitates more efficient channels for the lithium insertion and de-insertion while cycling. Furthermore, MWCNTs reduce the effect of volume expansion of Ge during lithium insertion, in fact Ge-MWCNTs composites have enough void spaces which allow for buffering of the volume change. In addition, the high electrical conductivity of MWCNTs improves composite material cyclability and capacity retention.

Summary

In summary, nano sized Ge nanocrystals and Ge-MWCNTs nanocomposite with highly uniform surface distribution of Ge nanocrystals have been synthesized via solvothermal technique. The composite of Ge-MWCNTs showed improved electrochemical performances compared to pure germanium, with a discharge capacity of ~ 1170 mAh g-1 after 10cycles. The good performances of the Ge-MWCNTs composite can be ascribed to the MWCNTs high conductivity and to the hindering effect on the agglomeration of Ge nanoparticles. Moreover MWCNTs provide enough space to buffer the volume changes during charge-discharge process. Finally, MWCNTs facilitate the uniform distribution of Ge nanoparticles, therefore creating more efficient channel for lithiation/de-lithiation of Ge.

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Germanium Nanocrystals-MWCNTs Composites as Anode Materials for Lithium Ion Batteries

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