A double-layered Ge/carbon cloth integrated anode for high performance lithium ion batteries

Abstract

A double-layered Ge coated carbon cloth composite was synthesized by electrodepositing Ge from an ionic liquid using carbon cloth as substrate. As an integrated electrode, the composite exhibits a high initial charge capacity of 1169 mA h g⁻¹ and retains 989 mA h g⁻¹ after 100 cycles at 300 mA g⁻¹.

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Lithium ion batteries (LIBs) are widely used in portable electronic devices, such as cell phones, digital cameras, and notebooks, and also have been considered as a potential power source for electric vehicles.¹ To meet the rapid development of such applications, LIBs with high energy density and long cycle life are required.^2,3 It is crucial to develop new electrode materials with higher capacity than current commercial ones.^4–6 Compared with graphite with a relatively low theoretical capacity of 372 mA h g⁻¹, group IV elements with higher theoretical specific capacity have attracted increasing attention.⁷ Si and Ge have possible gravimetric Li⁺ storage capacities of as high as 4200 and 1600 mA h g⁻¹ respectively. Although Ge has a relatively lower capacity than that of Si, it has excellent lithium-ion diffusivity (400 times faster than Si) and high electrical conductivity (10⁴ times higher than Si),⁸ thus making Ge an attractive electrode material for high-rate LIBs.

However, similar to Si electrode, the key drawback of the Ge electrode is the large volume change (up to 370%) during Li⁺ insertion/extraction,⁹ which leads to the pulverization of the electrode. To minimize the volume change, various forms of nanostructured Ge electrode materials, including nanowires,¹⁰ nanotubes,¹¹ nanoparticles¹² have been investigated and exhibit improved cycling performance. Another efficient strategy is to develop carbon-based Ge composites that not only provide a volume change buffer caused by Li⁺ insertion/extraction, but also improve the electrical conductivity of the Ge materials.^13–15 Very recently, carbon cloth has been used as conductive supports for electrode materials and shows a great potential in flexible electronic devices, due to its superior conductivity and high mechanical strength.^16–18 In addition, carbon cloth-based electrodes could also simplify the cell packing process without using any binders and current collectors, which results in improving the capacity and cycling stability of lithium ion batteries.¹⁹

Here we reported a novel double-layered Ge/carbon cloth integrated anode prepared by a one-pot electrodepositing Ge from an ionic liquid for the first time. In our experiment, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Emim]Tf₂N) was used as electrodeposition solution and GeCl₄ as Ge source. The success of the Ge deposition was proved by the cyclic voltammogram (CV) analysis, as shown in Fig. S1 (ESI).† On the cathodic scan, there are two reduction peaks at −1.08 V and −1.66 V, corresponding to the reduction of Ge(IV) to Ge(II) and Ge(II) to Ge(0), respectively. Typically, the deposition potential was maintained at −1.64 V for 60 min (see ESI for the Experimental details†). Fig. 1 presents the schematic illustration of the synthesis route for the composite. The growth mode for a metal M electrodeposited on a foreign substrate S follows eqn (1)

M⁺(aq) + S(s) + e⁻ → M–S(s) (1)

which can be explained by the binding energy of metal adatoms on foreign substrate (E_Meads–S) and the binding energy between the metal adatoms and the native metal substrate (E_Meads–Me).²⁰ When E_Meads–Me > E_Meads–S, the depositing metal segregates on the substrate to form islands by the “Volmer–Weber” growth mode mechanism. However, when E_Meads–Me < E_Meads–S or the two binding energies are similar, the depositing material preferentially forms uniform layer by layer films via a “Frank-van der Merwe” growth mode. In this experiment, due to the different binding energies of Ge⁴⁺ adatoms with carbon cloth and pre-deposited Ge, a porous inner layer of Ge island arrays was firstly deposited on the surface of carbon cloth fiber (“Volmer–Weber” growth), followed by the formation of a compact Ge film outer layer (“Frank-van der Merwe” growth).

Fig. 1 Schematic illustration of the synthesis route for the double-layered Ge film/carbon cloth composite. (a) Carbon cloth. (b) Carbon cloth fiber. (c) Ge island arrays growing on the carbon cloth fiber. (d) The growth of the compact Ge film on Ge island arrays. (e) The homogeneous double-layered Ge film electrodeposited on carbon cloth.

The SEM images of the pure carbon cloth are shown in Fig. S2 (ESI),† which exhibit three-dimensional (3D) architectures with smooth surface and uniform diameter around 9 μm. The growth of the Ge island arrays after deposition for 30 and 120 s is also shown in Fig. S2 (ESI),† indicating that the formation of the Ge island arrays is mainly at the initial stage. And it fits well with the current–time (I–t) curve of Ge electrodeposition at −1.64 V in Fig. S1b (ESI).† There is a quick current drop from 0 to 120 s when the growth of the Ge island arrays occurs. Fig. 2(a and b) clearly show the well-established texture of carbon cloth with homogeneous Ge film (around 1 μm) on it. The photographic image inset in Fig. 2a shows excellent flexibility of the Ge/carbon cloth composite. It can be rolled up with tweezers periodically, which makes it possible to be used as flexible electrode. Side-view SEM image of Ge film (as shown in Fig. 2c) indicates that the Ge film is double-layered with a compact outer layer and a porous inner layer. The porous inner layer is composed of Ge island arrays, while the outer layer is a compact film (around 0.3 μm) as shown in Fig. 2d.

Fig. 2 (a, b) SEM images of homogeneous Ge film growing on carbon cloth at different magnifications. Photographic image of the Ge/carbon cloth composite (inset in (a)) that can be rolled up with tweezers periodically. (c) Side-view SEM image of Ge film. (d) Plan-view SEM image of double-layered Ge film.

The double-layered Ge film is mainly composed of nanocrystalline germanium confirmed by the Raman spectra in Fig. S3 (ESI).† The Raman peak at 291 cm⁻¹ is corresponding to the nanocrystalline germanium,^21–23 while the small shoulder peak at 189 cm⁻¹ indicates that the Ge film is partly amorphous. Moreover, the two peaks at 1350 cm⁻¹ and 1588 cm⁻¹ correspond the D band (disordered induced phonon mode) and G band (graphite band) of carbon cloth. The XPS spectrum of Ge 3d is presented in Fig. S4 (ESI).† The bimodal curve at 28.9 eV and 29.6 eV are corresponding to Ge. The peak at 30.4 eV is attributed to GeO, while the peak at 32.5 eV is related to the GeO₂. The Ge⁰ signal from the Ge phase is weak, however the Ge⁴⁺ signal from the GeO₂ phase is strong, indicating that the surface of the electrodeposited germanium film is easily to be oxidized when exposed in air for several hours.

The electrochemical performance of the double-layered Ge coated carbon cloth composite was evaluated in comparison with the commercial Ge powder (sizes of 20–50 nm). In Fig. 3a, the cyclic voltammogram (CV) of Ge/carbon cloth electrode for the first four cycles was tested in the potential range of 0.01 to 2.00 V at 0.1 mV s⁻¹. The shape of the curve corresponding to the first cycle is different from the subsequent cycles in the cathodic branch, which mainly results from the formation of the SEI films. The reduction peaks at 0.14 V and 0.62 V could be assigned to the formation of Li–Ge alloy.²¹ In the anodic branch, the oxidation peaks at 0.36 V and 0.45 V are associated with Li⁺ extracting from germanium and carbon cloth.

Fig. 3 (a) Cyclic voltammogram for the Ge/carbon cloth integrated electrode from 0.01 V to 2.0 V (vs. Li/Li⁺) at 0.1 mV s⁻¹. (b) Charge and discharge capacities under different current densities of 300, 600, 1500, 3000 and 6000 mA g⁻¹, and then back to 300 mA g⁻¹. (c) Cycling performance of the electrode at 300 mA g⁻¹ from 0.01 V to 2.0 V (vs. Li/Li⁺). (d) Voltage profiles of the 1st, 5th, 10th, 50th of the electrode cycled at 300 mA g⁻¹.

Rate performance is shown in Fig. 3b. The Ge/carbon cloth electrode exhibits charge capacities of 1109, 984, 758, 504, and 400 mA h g⁻¹ at 300, 600, 1500, 3000 and 6000 mA g⁻¹, respectively. Especially, the reversible capacity can recover to 1030 mA h g⁻¹ when the current density is back to 300 mA g⁻¹.

The Ge/carbon cloth electrode is cycled at 300 mA g⁻¹ for 100 cycles, with the result shown in Fig. 3c and voltage profiles in Fig. 3d. The initial discharge and charge capacities are 1343 mA h g⁻¹ and 1169 mA h g⁻¹, respectively, with an initial 86.9% coulombic efficiency. Moreover a reversible capacity of 989 mA h g⁻¹ is achieved even after 100 cycles. In comparison, the commercial Ge powder shows a high initial capacity of 1351 mA h g⁻¹, but its capacity severely decreases after only 20 cycles. The excellent cycling stability of the Ge/carbon cloth electrode is attributed to the special structure: the inner layer of the Ge film is porous, which will withstand the volume change and maintain the mechanical stability of the Ge film; the compact outer layer keeps the electrolyte from entering into the porous inner layer by the SEI films forming on the outmost surface; the 3D interconnected carbon cloth framework enhances the physical connection and electrical contact with Ge film. The enhanced electrical conductivity of the Ge/carbon cloth electrode can also be explained by electrochemical impedance spectroscopy (EIS) in Fig. S5 (ESI).† Both the Nyquist plots are composed of two compressed semicircles and an inclined line. The semicircles are attributed to the surface film impedance in the high frequency and the charge transfer resistance in the middle frequency. The inclined line is attributed to the lithium diffusion impedance. Apparently, the Ge/carbon cloth electrode shows a much lower charge-transfer resistance than that of the Ge powder electrode (45 vs. 113 Ω), which could lead to a better electrochemical performance.

The cell of the double-layered Ge/carbon cloth nanocomposite was disassembled after 50 cycles and the electrode was investigated. The Ge film on carbon cloth remains integral without evident crack formation and peeling off from carbon cloth after 50 cycles (Fig. 4a). However, the Ge film is wrinkled, which results from the volume change during cycles. The rough surface could be resulted from the formation of SEI films on the surface (Fig. 4b). Compare with Ge powder, the stability of the double-layered Ge film is attributed to its special structure. As shown in Fig. 4c, when used as LIB electrode, the SEI films will mainly formed on the surface of the compact outer layer, which will reduce the electrolyte inserting with Li⁺ into the electrode materials and side reactions between germanium and the electrolyte. The porous inner layer is composed of Ge island arrays, which will provide enough space to buffer the volume change. However, the common Ge powder electrode will be easy to pulverise and leave from the current collectors.

Fig. 4 (a, b) SEM images of the Ge film electrodeposited on carbon cloth after 50 cycles at 300 mA g⁻¹ at different magnifications. (c) Ge electrode failure mechanisms of (1) Ge powder electrode by common method and (2) the double-layered Ge film/carbon cloth electrode obtained in this work.

In conclusion, we prepared a 3D double-layered Ge/carbon cloth electrode by a one-pot electrodeposition from [Emim]Tf₂N. The flexible Ge/carbon cloth composite can be directly used as integrated self-supported and binder-free anode for LIBs. And it presents excellent electrochemical performance with initial discharge and charge capacities of 1343 mA h g⁻¹ and 1169 mA h g⁻¹, respectively. A reversible capacity of 989 mA h g⁻¹ can be obtained after 100 cycles at 300 mA g⁻¹. The integrated Ge/carbon cloth electrode not only ensures high and stable capability, but also possesses excellent flexibility, which makes it possible to be used as flexible electrode.

Acknowledgements

This work was supported by National Key Basic Research Program of China (2016YFB0100104), National Natural Science Foundation of China (No. 21276257, 91534109, 91434203) and “Strategic Priority Research Program (XDA09010103)” of Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, CV curve, SEM figures, Raman and XPS spectra. See DOI: 10.1039/c6ra12671e

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