Article pubs.acs.org/cm
Honeycomb-like Macro-Germanium as High-Capacity Anodes for Lithium-Ion Batteries with Good Cycling and Rate Performance Jianwen Liang,† Xiaona Li,† Zhiguo Hou,† Tianwen Zhang,† Yongchun Zhu,*,† Xuedong Yan,§ and Yitai Qian*,† †
Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China § Ningbo Veken Battery Company Inc., West Bonded Zone, Ningbo, Zhejiang 315000, P.R. China S Supporting Information *
ABSTRACT: Macro-Ge powder has been synthesized with a novel hydrothermal reduction of commercial GeO2 at 200 °C in an autoclave. The obtained macro-Ge product demonstrates a honeycomb-like macroscopic network structure with a high tap density of 2.19 g cm−3. As for the anode material of lithium ion batteries, the macro-Ge electrode exhibits 1350 mAh g−1 at the current rate of 0.2 C and with 64% capacity retention over 3500 total cycles at 1 C. The macro-Ge contains a honeycomb porous structure, which allows for a high volumetric capacity (∼3000 mAh cm−3). Moreover, the symmetrical and asymmetric rate behaviors also provide its excellent electrochemical property. For example, the macro-Ge electrode can be rapidly charged to 1130 mAh g−1 in 3 min (20 C) and 890 mAh g−1 in 90 s (40 C) using the constant discharge mode of 1 C. Furthermore, the Ge electrode still maintains over 1020 mAh g−1 at 1 C for 300 cycles at the high temperature (55 °C) environment. When coupled with a commercial LiCoO2 cathode, a 3.5 V lithium-ion battery with capacity retention of 91% (∼364 Wh kg−1) over 100 cycles is achieved. These outstanding properties may be attributed to the honeycomb structure, for which the porous architectures supply the high efficient ionic transport and buffers the volume change during the lithiation/delithiation processes. Moreover, with bulk frameworks it ensures the high tap density and further improves the energy density. It is supported that the macro-Ge acts as attractive anode materials for further application in rechargeable lithium ion batteries.
1. INTRODUCTION Rechargeable lithium-ion batteries (LIBs) are key electrical energy storage devices for next generation electricity storage and supply. To satisfy the increasing demand, it is necessary to develop LIBs with higher energy density, higher power density, and more desirable stability.1,2 From the viewpoint of energy density, group IVA elements are the most promising materials, especially Si and Ge, which can deliver theoretical capacities of 4200 mAh g−1 (volumetric capacity of 8344 Ah L−1) and 1600 mAh g−1 (volumetric capacity of 7366 Ah L−1), respectively.3−5 Compared with Si-based materials, Ge exhibits both a higher electronic conductivity (104 times) and a higher lithium ion diffusivity (more than 400 times higher than silicon at room temperature: 1.41 × 10−14 cm2 s−1 for Si and 6.51 × 10−12 cm2 s−1 for Ge),6,7 which can be expected to provide better rate performance and cycling stability. In essence, Ge has attracted less attention than Si recently because of its expensive preparation process. However, the cost issue can be surmountable by mass production and advancements in the process technology as a result of the abundance of Ge in the Earth’s crust.8 In terms of the synthesis in Ge, one of these methods was chemical vapor deposition (CVD) of germane (like Ge2H6), chlorogermanes, or phenylgermanes at >400 °C with the presence of gold nanocrystals seeded.9,10 Nanostructure Ge materials were also prepared by the typical thermal degradation © 2015 American Chemical Society
of organogermane, GeI2 or GeCl4 precursor in organic solvent.11−14 Ge nanoparticles were produced by oxidation of zintl alloy such as Na12Ge27 in a glovebox or on a vacuum line.15 With regard to the using of GeO2 precursors in synthesizing of Ge, major attention has been paid to the preparation in the thermal reduction by H2,16,17 Mg,18 and Zn19 at the temperature higher than 450 °C. Although many strategies have been proposed to prepare the Ge materials, those procedures generally need an expensive catalyst such as gold nanocrystals, high temperature, or complicated procedures, or they employ expensive Ge precursors such as GeH4, GeI2, and diphenylgermane (C12H10Ge). Thus, for the desire of lowering the cost of Ge-based electronic devices, facile synthesis of Ge by a new effective method is still a challenge to overcome. On the other hand, similar to silicon anode, the practical usage of Ge as anode material still suffers from a huge volume change (∼300%) during the Ge lithiation−delithiation process. These extreme volume changes can cause cracking, pulverization, loss of electrode contact, and continuous solid−electrolyte interface (SEI) films formation in the electrode. Thus, many approaches have been explored to improve the electrochemical Received: April 25, 2015 Revised: May 12, 2015 Published: May 12, 2015 4156
DOI: 10.1021/acs.chemmater.5b01527 Chem. Mater. 2015, 27, 4156−4164
Article
Chemistry of Materials
2. EXPERIMENTAL SECTION
performance of the Ge anode, such as the design of special morphology of Ge nanoparticles20−22 and the preparation of Ge/carbon23−26 or Ge-alloy27−29 nanocomposites. Nanosized Ge can provide sufficient space to tolerate the large volume change, and the employing of carbon or alloy metal can improve the conductivity between Ge particles. However, preparation of nanosized Ge either requires expensive Ge precursors, a special device, or involves complicated processes; again the issues for cost and scaling up remain. Furthermore, achieving a high tap density by using a micron framework is particularly important for fabrication of high-energy LIBs.30 Thus, Ge microparticles are more promising as electrode materials toward practical application because they lead to higher volumetric energy density when compared to nanoparticles. Significantly, the macro-size of Ge is commercially available or easy to large scale preparation. Recently, Tobias Placke and co-workers showed that macroporous-sized Ge particles can have excellent charge−discharge cycling stability at a certain condition.31 Their macroporous Ge material exhibits a capacity retention of 96.6% at 0.1 C for 200 cycles and 717 mAh g−1 retention when the rate is up to 5 C. In our present work, a hydrothermal reduction process to prepare silicon nanomaterials by using silica sol or silica aerogel has been proposed.32 Such a hydrothermal reduction process can retain the original size of the precursor with porous structure formation. Inspired by the similarity between the silicon and germanium in chemical properties and synthesized strategy,11,14,18,33−36 we further develop the hydrothermal reduction process to prepare the macro-Ge powder by using a commercial bulk GeO2 precursor at a low temperature (200 °C). Compared with the traditional synthesis route, the hydrothermal reduction of the GeO2 strategy has several advantages. First, the reaction temperature for the hydrothermal reduction process is much lower than that for typical thermal reduction reactions; on the other hand, the fabrication process is a one-step synthetic reaction using an environmentally benign and cost-effective approach, which allows for the production of the Ge powder on a large scale. The obtained product demonstrates a honeycomb-like macroscopic network structure with a high tap density of 2.19 g cm−3. As anode material in LIBs, the macro-Ge delivers not only a high reversible capacity of 1350 mAh g−1 (corresponding to ∼3000 mAh cm−3) at the current rate of 0.2 C (1 C = 1600 mA g−1) and a superior stability (64% capacity retention even over 3500 total cycles at 1 C). Moreover, the performance of both galvanostatic and asymmetric rate also exhibits its excellent electrochemical performance. For instance, the macro-Ge electrodes can be rapidly charged to 1130 mAh g−1 in 3 min (20 C) and 890 mAh g−1 in 90 s (40 C) using the constant discharge mode of 1 C. Even at a high temperature (55 °C) environment, the macro-Ge electrode still maintains over 1020 mAh g−1 at 1 C for 300 cycles. When coupled with a commercial LiCoO2 cathode, a 3.5 V full lithium-ion battery with excellent storage properties (with capacity retention of 91% (∼364 Wh kg−1) over 100 cycles) is further demonstrated. Thus, the novel hydrothermal reduction strategy was demonstrated and proved to be an effective way to synthesize the macro-Ge anodes. Such an approach greatly reduces the preparation cost, not only avoiding expensive Ge precursors but also getting rid of the special device or complicated processes. This advanced macro-Ge electrode with high capacity and good stability provides a promising Ge anode for LIBs with high energy density and superior stability.
Materials. GeO2, Mg (99%, 100−200 mesh powder), and HCl (37%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Commercial LiCoO2 electrode materials came from Ningbo Veken Battery Company (5th Gangxi Avenue, West Bonded Zone, Ningbo, China). Hydrothermal Synthesis of Macro-Ge Particle. The macro-Ge was prepared by reduction of commercial GeO2 with magnesium and water according to a hydrothermal reduction reaction under the stainless autoclave. Typically, 0.8 g of GeO2, 1.5 g of deionized water, and 2 g of Mg powder were mixed and added into a 20 mL stainless autoclave before sealed. Subsequently, the autoclave was maintained at 200 °C for 5 h and cooled to room temperature. The as-prepared samples were immersed in hydrochloric acid (1 mol L−1) for 1 h to remove impurities. The resultant powder was washed with deionized water and ethanol and dried overnight at 60 °C in a vacuum oven to evaporate the rest of the solvent to collect Ge. Characterization. The morphology of the reaction product was characterized by scanning electron microscopy (SEM, JEOL-JSM6700F); transmission electron microscopy (TEM) images were digitally acquired using a field emission Hitachi H7650 TEM operated at 100 kV with macro-Ge disperse on 200 mesh lacey-carbon copper TEM grids. X-ray diffractometer (XRD) was performed on a Philips X′ Pert Super diffract meter with Cu Kα radiation (λ = 1.54178 Å). The Brunauer-Emmett-Teller (BET) surface area and Barrett−Joyner− Halenda (BJH) pore distribution plots were measured on a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. The tap density of the Ge powder was measured with a Powder Autotap Density Meter (JT-1, Chengdu Jingxin Powder Analyze Instrument Co., LTD). First, a stainless steel cylinder is filled with the as-prepared Ge powder of known weight (marked as m) and mounted onto the Autotap instrument. The Autotap is then programmed to automate 1000 taps with the rate of one tap per second. Then, the volume of the powder (marked as V) is recorded, and the tap density (marked as TD) is calculated based on the formula of TD = m/V and expressed in g cm−3. The tap density of the Ge powder is averaged based on three repeated experiments. Electrochemical Measurement of the Macro-Ge. The electrochemical properties of the macro-Ge electrodes were measured with coin-type half cells (2016 R-type) which assemble under an argonfilled glovebox (H2O, O2 < 1 ppm). A working electrode was prepared by mixing the macro-Ge material, super P carbon black, and sodium carboxymethylcellulose (Na-CMC) binder in a weight ratio of 60:20:20 in water solvent. The slurry was pasted onto a Cu foil and then dried in a vacuum oven at 80 °C for 12 h. The active material density of each cell was determined to be 0.8−1.4 mg cm−2. A metallic Li sheet was used as counter electrode, and 1 M LiPF6 in a mixture of ethylene carbonate/dimethylcarbonate (EC/DMC; 1:1 by volume) and 5 wt % fluoroethylene carbonate (FEC) were used as the electrolyte (Zhuhai Smoothway Electronic Materials Co., Ltd. (China)). Galvanostatic measurements were made using a LANDCT2001A instrument at room temperature that was cycled between 0.005 and 1.50 V versus Li+/Li at a different current density from 0.16 to 80 A g−1. The AC impedance spectra were recorded with a CHI660e electrochemical station by applying an AC voltage of 5 mV in amplitude in the frequency range of 0.01 Hz to 100 kHz at room temperature. The obtained spectra were analyzed using ZView software. Except for the high temperature measurement, all electrochemical performance data were measured at room temperature. Electrochemical Measurement for a Ge-LCO Full Cell. The electrochemical analysis was carried out at room temperature with a CR2016 coin-type full cell using an LCO electrode as cathode in the potential range between 2.50 and 4.20 V. The LCO cathode electrode was prepared by mixing the commercial LCO material, carbon black, and poly(vinyl difluoride) (PVDF) binder in a weight ratio of 8:1:1. NMethyl-2-pyrrolidone (NMP) was used as the solvent to form homogeneous slurry. The resulting slurry was coated onto the Al foil and dried at 120 °C for 12 h. After drying and pressing, the Al foil was cut into dish (12 mm in diameter) with LCO activity material loading 4157
DOI: 10.1021/acs.chemmater.5b01527 Chem. Mater. 2015, 27, 4156−4164
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Chemistry of Materials
cm−1 further confirms the fabrication of crystalline Ge.39 The yield of this Ge material is above 40%. Figure 2 is the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-
of 7−8 mg cm−2. Prior to their assembly in full cells, the macro-Ge electrodes were electrochemical prelithiation in half-cell configuration (vs metal Li) for one cycle. The coin full cell was assembled in a glovebox filled with argon gas similar to that of the Ge anode with the LCO electrode as cathode and the prelithiation Ge electrode as anode.
3. RESULTS AND DISCUSSION 3.1. Hydrothermal Synthesis of the Macro-Ge Powder. Commercially available macro-sized GeO2 is employed as the Ge source. As shown in Figure S1, the initial GeO2 particle is a macro-sized structure with a rough and angular surface. Similar to our previous reports32,37 in hydrothermal synthesis of Si, the hydrothermal reduction process of GeO2 is promoted by the active H intermediates, which is similar to the Clemmensen reaction in traditional organic synthesis.38 In the Clemmensen reaction, active H intermediates generated from Zn and HCl transform methylene from carbonyl. Afterward, followed by removal of the impurity via an acid etching process, macro-Ge particles are obtained. The phase of the precursor and products obtained at different steps in the synthesis process were characterized by Xray diffraction (XRD). The XRD pattern of the GeO2 precursor is shown in Figure 1a. All the peaks can be indexed to the pure Figure 2. Morphology characteristic of the macro-Ge by using the hydrothermal preparation: a) typical SEM image, b) enlarged image, and c) TEM image of macro/nanoporous Ge. d) Barrett−Joyner− Halenda (BJH) distribution of the resulting macro-Ge powder.
prepared Ge materials. From the SEM image (Figure 2a), one can see that the as-prepared Ge is composed of honeycomb-like morphology with a macroporous structure. The interconnected feature of Ge is clearly exhibited in the high magnification SEM image (Figure 2b) and TEM image (Figure 2c). Nitrogen adsorption (Brunauer−Emmett−Teller, BET, Figure S3) measurements indicate that the specific surface area of the macro-Ge is 3.3 m2 g−1. Barrett−Joyner−Halenda (BJH, Figure 2d and Figure S4) analyses of the nitrogen adsorption curves indicate that the macro-Ge possesses nanopores (several to tens of nanometers in diameter) and macropores (hundreds of nanometers in diameter). The results above demonstrate that honeycomb-like macroporous Ge particles consist of several interconnected porous and aggregation structures. The tap density of the macro-Ge powder was estimated to be 2.19 g cm−3, which is higher than that of the commercial graphite anode (1.3 g cm−3) and the previous reports40 of the macrosized Ge anode (1.8 g cm−3). 3.2. Electrochemical Performance of Macro-Ge Anodes in Lithium Batteries. In order to evaluate the honeycomb-like macro-Ge as a potential anode material for lithium ion batteries, CR2016 type coin-cells with lithium foil as a counter electrode were prepared. Figure 3 shows the cyclic performance of the macro-Ge electrodes at current density of 1 C after the first five cycles activated at 0.2 C (1 C = 1600 mA g−1). The voltage profiles of the macro-Ge at the first five cycles during the electrode activation process are shown in Figure S5. The first discharge curve displays a long flat plateau about 0.5 to 0.1 V, which corresponds to the lithiation process of crystalline Ge to form different LixGe alloys.41 Subsequently, the discharge and charge curves show the characteristics of the amorphous Ge anode. The electrode delivers a capacity of 1512 mAh g−1 and 1263 mAh g−1 for the first discharge and charge capacity, which corresponds to an initial Coulombic efficiency of 84%. Afterward, the macro-Ge electrode affords superior storage performance with reversible capacity about 1350 mAh
Figure 1. XRD patterns of products obtained at different steps during preparation: a) commercial GeO2 percursor, b) the reacted specimens after hydrothermal reduction at autoclave in 200 °C for 5 h, and c) the resulting Ge product.
hexagonal GeO2 (JPCDS 85-1515). After hydrothermal reduction at an autoclave in 200 °C for 5 h, XRD analysis (Figure 1b) confirmed the main component of MgO, Mg(OH)2, and Ge in the reacted specimens. A trace amount of Mg2GeO4 was also detected from the XRD pattern, which might be due to the reaction between GeO2 and Mg(OH)2. The composite reacted specimens were then immersed in a 1 M HCl solution for several hours to remove the impurity. The XRD pattern of the resulting Ge product is shown in Figure 1c. No GeO2 phase was detected during or after the process. The peaks at 27.5°, 45.4°, 53.8°, 66.2°, and 73.0° can be completely indexed to the standard cubic phase Ge with a calculated lattice constant of a = 5.64 Å, which is consistent with that of crystalline Ge (JCPDS No. 89-2768; Fd3m, a = 5.6568 Å). Further structural information is provided by the Raman spectra in Figure S2. The strong Raman peak located at ∼300 4158
DOI: 10.1021/acs.chemmater.5b01527 Chem. Mater. 2015, 27, 4156−4164
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Chemistry of Materials
Figure 3. Electrochemical performances of macro-Ge anode. a) Typical galvanastatic discharge−charge curves of the cell with Ge anode in the potential region of 0.005−1.5 V versus Li+/Li at a current density of 1C after 0.2 C activation; b) cycling property and Coulombic efficiency of the cell with macro-Ge anode at the constant current density of 1C after 0.2 C activation [■ (blue) as discharge capacity (Qd), ● (red) as charge capacity (Qc), and ▲ as Coulombic efficiency (Qc/Qd)]; and c) cycling property at 1 C over 3000 cycles.
g−1 at 0.2 C, which is very close to the theoretical capacity of Ge (∼1600 mAh g−1). Moreover, the volumetric capacity was determined to be ∼3000 mAh cm−3, which is much higher than that of the commercial graphite anode (483 mAh cm−3). Such a high initial Coulombic efficiency and reversible capacity value indicates high activation of the macro-Ge for lithiation and delithiation. Figure 3a shows the typical voltage-capacity profiles for the macro-Ge electrodes after 1, 50, 100, 200, 500, 1000, 2000, and 3000 cycles at 1 C. With the cycle number increasing, the discharge/charge voltage curves overlap well with each other and decrease slightly, indicating stability with the lithiation/delithiation process. The reversible Li charge/ discharge capacity and the Coulombic efficiency of the macroGe electrode versus cycle number are plotted in Figure 3b. The macro-Ge electrode exhibits a relatively stable reversible lithium capacity of ∼1180 mAh g−1 at 1 C after activation and retained a quite high capacity value of ∼1040 mAh g−1 after 300 cycles, with 88% capacity retention. The Coulombic efficiency increases to 99% after several cycles and stabilizes at ∼100% in later cycles. Significantly, even more than 3500 deep cycles, the specific capacity still remains a high value of ∼755 mAh g−1 (64% capacity retention, Figure 3c), which is more than two times the theoretical capacity of a commercial graphite anode (372 mAh g−1). The fact that the capacity only degrades 1.02% per 100 cycles over 3500 total cycles indicates the superior stability of the macro-Ge electrode. On the other hand, in virtue of the high packing density of the electrode materials, our macro-Ge with a tap density of 2.19 g cm−3 is also expected to have a high volumetric energy density (Figure S6 and Figure 3c). Moreover, there are some undulations in the specific capacity values during the cycling test, which do not effect neither the average value of the specific capacities nor the
ability of cycling. The slight capacity undulations might result from the temperature variation both inside of the cell and the measurement environment, which is similar to our previous reports.17,32 Moreover, the rate capability for the macro-Ge anode is evaluated using galvanostatic charge−discharge measurements with increasing the current density from the low current density of 0.1 C to a high current density of 50 C over the potential window of 0.005−1.50 V. Typical charge and discharge profiles are shown in Figure 4a. In good agreement with the typical charge/discharge behavior of Ge, the discharge curves have a dominant plateau at about 0.3 V and the charge plateau at about 0.5 V. With increasing current density, the discharge voltage plateau only decreases slightly, indicating the small polarization of
