Guo-An Li,
Wei-Chin Li,
Wei-Chung Chang and
Hsing-Yu Tuan*
Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, Republic of China. E-mail: hytuan@che.nthu.edu.tw; Fax: +886-3-571-5408; Tel: +886-3-572-3661
First published on 10th October 2016
Germanium oxide (GeO2) nanoparticles were synthesized with a nearly 100% production yield in a nonionic reverse micelle system at ambient temperature. The procedure is a facile and energy saving strategy for producing germanium oxide nanoparticles with ultra large throughput. As-prepared GeO2 nanoparticles can be directly used as anode materials without any post-treatment or other supplementary additives for lithium ion batteries. GeO2-anodes exhibited good electrochemical performance in terms of both gravimetric and volumetric capacity. The GeO2 anodes have a reversible capacity of approximately 1050 mA h g−1 at a rate of 0.1C, close to its theoretical capacity (1100 mA h g−1), and good rate capability without severe capacity decade. The volumetric capacity of the GeO2 anodes reaches 660 mA h cm−3, which is higher than the performance of commercial graphite anode (370–500 mA h cm−3). Coin type and pouch type full cells assembled for electronic devices applications were also demonstrated. A single battery is shown to power LED array over 120 bulbs with a driving current of 650 mA. Based on the above, the micelle process of GeO2 nanoparticle synthesis provides a possible solution to high-capacity nanoparticles' scalable manufacturing for lithium ion battery applications.
Germanium dioxide, also called germania, is much less expensive than germanium, and widely used in optical fibers, polymer catalysts, and also as a component of waveguides where it is capability of modulating the index of refraction.17,18 In general, various metal oxide nanostructures exhibit excellent electrochemical performance in lithium-ion batteries.19–22 GeO2 reacts with lithium ion and transforms irreversibly into germanium nanoparticles and Li2O matrix during beginning of reduction process, and then germanium nanoparticles react reversibly with lithium ion through alloying mechanism for the following lithiation/delithiation process.23,24
GeO2 + 4Li → Ge + 2Li2O |
Ge + 4.4Li ↔ Li4.4Ge |
Based on the above reaction, GeO2 has a theoretical capacity of 1100 mA h g−1 (4653 mA h cm−3) which is comparable to germanium's performance. However, the practical capacity of GeO2 is not well-maintained. The rapid capacity loss of the GeO2 electrode fabricated by Brousse et al.25 dropped from 740 to 225 mA h g−1 after 10 cycles is attributed to the ∼230% volume change during electrochemical test. Recently, lots of efforts have been made to address situations to obtain progressive improvement. GeO2 electrode maintains cycling stability with the aid of carbonaceous additives to form composites, which effectively alleviate the stress from volume variation during lithium intake/removal.26–30 In the meanwhile, supplementary additives offer additional electron transfer pathway, which represent kinetics improvement related to rate capability for LIBs. Another strategy to ameliorate GeO2 poor performance is size-controlled and architecture-design of the active material. Size and morphology of active material indeed play an important role in electrochemical performance for LIBs. Various nanostructures of GeO2, such as nanotube,31 nanoparticle,32,33 3-D porous nanostructure,34 exhibit a high reversible capacity and capacity retention with slightly capacity fade, which implies the alleviation of stress from Li intercalation/de-intercalation to maintain electrode integrity. They also perform well in rate capability determined by the kinetics of electron conductivity and lithium diffusivity. On the basis of time constant τ, kinetics strongly depend on the diffusivity length (τ ≈ L2D−1). However, these methods used to synthesize GeO2 nanostructures, such as thermal oxidation,35,36 template-directed method,37–39 and thermal evaporation,40 usually need harsh experimental conditions or complicated manufacturing process, which not only produce unnecessary impurities harmful to environment but also increase the manufacturing cost. Those synthetic methods for GeO2 might not be suited for industrial-scale production. Besides, GeO2 nanomaterials were usually incorporated with carbon additives, such as carbon nanotube, graphene, 2-D matrix for assembling into LIBs. The batteries gain stable cycling life or increase specific capacity, but sacrifice the overall energy density due to the addition of extra volume and weight to electrode. Therefore, a suitable strategy is required to fabricate GeO2 nanoparticles on the purpose of sustainability and scalability.
Herein, we report the synthesis of high quality hexabranched GeO2 nanoparticles at room temperature with a nearly 100% yield via an optimized sol–gel process (reverse micelle). With well controlling the oil/water phase, surfactant/co-surfactant ratio and choosing the right germanium source, the well-defined GeO2 nanoparticles can be obtained abundantly. In addition, as-prepared GeO2 nanoparticles were used as anode materials for lithium ion batteries without any post-treatment. The electrochemical results demonstrate high capacity and quite stable capacity retention. Finally, coin type and pouch type full cells assembled with the GeO2 as anode and Li(NiCoMn)O2 as cathode are fabricated and evaluated. Pouch type full cells were utilized to power light-emitting-diodes (LEDs) over 120 bulbs with a large current (∼650 mA).
In the fabrication of coin-type and pouch-type full cells, the Li(NiCoMn)O2 electrode with a loading mass of 21.12 mg cm−2 was to replace lithium metal as reference/counter electrode, and the anodes GeO2 nanoparticles remained the same loading mass. For perfect full cell assembly, the electrochemical performance of the cathode Li(NiCoMn)O2 was evaluated by half-cell measurement. The slurry for cathode electrode was prepared by mixing active materials (Li(NiCoMn)O2, 94.5 wt%) with 3.5 wt% of Super-P and 2 wt% of PVDF (polyvinylidene fluoride) binder dispersed in NMP (N-methyl-2-pyrrolidone) solvent. From the Fig. S7,† the areal capacity for the cathode electrode was ∼3.3 mA h cm−2 and the working potential of Li(NiCoMn)O2 vs. Li was observed at 3.8 V. For pouch type full cell assembly, GeO2 anode, membrane, and Li(NiCoMn)O2 cathode were stacked orderly in the Al-laminated film after welding metallic strip terminal and then filled with electrolyte. For the sake of electrolyte soaking entirely, pouch type full cell were rest for half hour after cell sealing. All the electrochemical performance of the GeO2-based lithium ion batteries were evaluated using Maccor Series 4000 instruments.
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Fig. 1 The illustration of experimental design of GeO2 nanoparticles synthesized in microemulsion system at room temperature. |
The crystal structure of GeO2 nanoparticles was investigated by X-ray diffraction analysis, as shown in Fig. 2(b). The XRD pattern perfectly matches the pure hexagonal phase structure of α-GeO2 (JCPDS no. 36-1463 with unit cell constants a = 4.985 Å and c = 5.648 Å), and it is obvious that no impurity phase was observed. Further surface and morphology characterization of GeO2 are provided by SEM images and the shape of product developed in this scale-up experiment design is hexabranched without transformation (Fig. 2(c)). The average length and width of the GeO2 nanoparticles are determined to 200 ± 20 nm and 150 ± 25 nm based on statistics analysis over 500 nanoparticles measured from SEM images. TEM image and its corresponding selected area electron diffraction (SAED) of well-defined GeO2 nanoparticle are given (Fig. 2(d) and (e)). Because of the inherent structural growth, the (101), (111) and (102) planes can be determined by diffraction perpendicular to the long axis. In addition, EDX analysis revealed the composition of GeO2 nanoparticles showing exact atomic ratio shown in Fig. 2(f).
To evaluate the electrochemical performance of GeO2 nanoparticles as anode material for LIBs, the product was assembled into coin type half-cell (CR2032) with Li metal as counter and reference electrode. First of all, the galvanostatic charging/discharging measurement was implemented at the current rate 0.1C (1C = 1.1 A g−1) with working voltage window of 0.01 V–1.5 V at room temperature. The cycling life curve in the Fig. 3(a) reveals that first charge and discharging capacity of the GeO2 nanoparticles are 1948 mA h g−1 and 1074 mA h g−1, respectively, corresponding to the coulombic efficiency of 55.1%. Presumably, the large irreversible capacity loss (874 mA h g−1) in the first cycle is attributed to the formation of Li2O, usually found in other metal oxide species such as silicon oxide.43,44 According to the previous research,23,45–47 it is believed that metal nanoparticles formed in delithiation process have catalytic property to decompose Li2O, which improves the coulombic efficiency of initial charging/discharging test. After the following cycles, the average reversible capacity of GeO2 nanoparticles was 1050 mA h g−1 with the excellent capacity retention of 87% based on the fifth cycle capacity (1105 mA h g−1). The stability of cycling life curve represent the electrode prepared by hexabranched GeO2 nanoparticles could tolerate the stress torture from huge variation during lithium intake and removal. Fig. 3(b) depicted the voltage profiles of the GeO2 anode in the first cycle. During the charge process, the voltage decreases dramatically from the open circuit voltage to approximately 0.7 V, and the plateau region show up, and then the voltage further decreases slowly until around 0.15 V. In the discharge profile, the plateau can be clearly observed from 0.3 V to 0.6 V. To clearly understand the reaction mechanism of GeO2 with lithium, differential capacity profiles shown in the Fig. 4(a) and (b) were derived from the voltage profiles of the first to 100th cycle in Fig. 3(a), and these peaks indicate various electrochemical reactions related to insertion/extraction of Li ions. Observed from the Fig. 4(a), there exist a distinct peak located near 0.5 V in the first charge cycle and vanished in the subsequent cycle, which probably present formation of SEI (solid electrolyte interface) layer and Li2O in irreversibility.33,48 At the following lithiation processes, small peak at approximately 0.13 V correspond to the Li–Ge alloying reaction. Correspondent Li–Ge de-alloying reaction is indexed to the small hump (0.35 V) in the delithiation process. Apparently, over several tens of times cycle test (Fig. 4(b)), the small peak around 0.37 V in the reduction sweep showed up, which is associated with formation of lithium-rich germanium alloys. Furthermore, the peaks at 0.62 V in reduction sweep and 0.7 V in oxidation sweep can be indicated to the redox reaction of germanium. However, the redox reaction reversibility is gradually decreasing, which may be a cause of capacity fading. These results are consistent with those reported research for cyclic voltammetry of GeO2.49,50 In reality, many electronic applications require high current operation. To confirm the feasibility of GeO2 in practical application, high rate test of GeO2-based batteries was executed. As shown in Fig. 5(a), current rate 1C with respect to 1100 mA h g−1 is employed and the discharge capacity is from 982 mA h g−1 in 2nd cycle to 856 mA h g−1 in 50th cycle with 87% capacity retention. Except 1C test, multi-rate with different operating current rates and its voltage profiles are depicted to examine the rate capability of GeO2 anode (Fig. 5(b) and (c)). From Fig. 5(b), the plateau isn't obvious once cycled at high rate, which implies the difficulty in lithium insertion. At a high current density test, the lithium insertion into active material site is rate-determining step, which means the kinetic limitation hinder the further lithium ions react with GeO2 nanoparticles.51,52 The GeO2 anode exhibits discharge capacities of 1250 mA h g−1, 1100 mA h g−1, 1000 mA h g−1, 855 mA h g−1, 590 mA h g−1, 420 mA h g−1, and 250 mA h g−1, corresponding to the various current densities of 0.1C, 0.5C, 1C, 2C, 4C, 6C, and 8C, respectively shown in Fig. 5(c). Finally, the specific discharge capacity of about 1170 mA h g−1 is recovered fast when the rate is returned to 0.1C again after 35 cycles. The additional electrochemical performance of GeO2 nanoparticles are provided in the ESI.† GeO2-based batteries employ different composition electrolytes system showing diverse electrochemical performance. As shown in Fig. S1,† FEC/DMC electrolyte system possesses more stable capacity retention and high rate performance than that of EC/DMC electrolyte system, which is perhaps due to formation of good SEI layer on the electrode surface.53–55
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Fig. 3 (a) Charge/discharge cycle performance of GeO2 anode at a 0.1C rate between 0.01 V and 1.5 V. (B) Voltage profiles of 1st and 100th cycle at a 0.1C rate. |
To further understand the interfacial electrochemical behavior of GeO2 electrodes, electrochemical impedance spectroscopy (EIS) analysis was performed at frequencies from 10 kHz to 10 mHz. From the Nyquist plots (Fig. 5(d)), the diameter of semicircle at high frequency region became small after charging/discharging test, which implies that transfer electron resistance reduces possibly due to the composition change of electrode such as formation of Li2O matrix.56–58 As for the inclined straight line at low frequency region, the slope of the line is relevant to lithium ion diffusivity which shows the difficulty for lithium ion immigration after 100 cycles.
In order to know the situation of composition and appearance of GeO2 nanoparticles after electrochemical test, after 100th cycling in galvanostatic charge/discharge cycles at 0.1C rate was disassembled and cleaned with DEC solvent to remove residual byproducts. Fig. 6(a) demonstrates the intuitive evidences for before/after 100 cycling test in coin type cell, from the ex situ XPS measurement (Fig. 6(b)) of GeO2 anode. It is obvious that the peak indicated (32.4 eV) corresponds to the Ge–O bonds. After cycling 100th cycles, the peak representing Ge–O bonding disappear, which is due to the full conversion of GeO2 to form germanium nanoparticles and Li2O. The single peak observed at 29.8 eV is well indicated to the elemental germanium. From XRD patterns shown in Fig. 6(c), there are two obvious peaks indexed to the (100) and (101) crystalline planes of GeO2 nanoparticles with good crystallinity for fresh electrode. After cycled 100 times, the crystalline GeO2 became amorphous, and showed no obvious peak from XRD pattern. Consequently, these evidences demonstrate that active material in the beginning is crystalline GeO2 and finally turns to the amorphous germanium at the following cycles.
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Fig. 6 (a, b) Photographs of GeO2 anode color before/after 100th cycling intuitively (scale bar = 1 cm). (c) XRD pattern, (d, e) XPS analysis of GeO2 anode before/after 100th cycling. |
Volumetric capacity of active material is another essential consideration to evaluate the feasibility of the material applied to industrial development.59–61 According to the characteristics of GeO2 anode including gram-scale and great electrochemical properties analyzed in above experiments, the volumetric capacity of the GeO2 is around 660 mA h cm−3 on the basis of compress density 0.6 g cm−3 (active material weight, electrode area, and the thickness of the GeO2 electrode are 0.5 mg, 0.95 cm2, and ∼10 μm, respectively). This performance is higher than commercialized graphite anodes (370–500 mA h cm−3).62–64
Coin type and pouch type full cell were fabricated combined with cathode ternary material Li(NiCoMn)O2 to evaluate the feasibility of GeO2 nanoparticles toward its application. Fig. 7(a) show the cycle performance of GeO2 at rate of 0.1C in CR2032 coin type cells exhibited discharge capacities 1300 mA h g−1 in the first cycle between 2.5 V and 4.2 V and have a capacity of 950 mA h g−1 after 100 cycle. The subsequent slightly capacity fade is due to the unbalanced capacity ratio of cathode/anode and highly irreversible capacity of anodes. GeO2-based coin type full cell also perform well at the rate of 1C with reversible capacity of 600 mA h g−1. As for the pouch cell, the size of electrodes used in assembly is approximately 8 cm2 × 5 cm2, and the anode size is bigger to prevent Li-dendrite inside the cell. The mass loading of cathode and anode is the same as which is in CR2032 coin type cell. The total capacity and specific capacity are shown as Fig. 7(d). The capacity offered by the single pouch type battery at the rate 0.1C is 150 mA h (1878 mA h g−1) in the first cycle, and the subsequent cycles show reversible capacity of approximately 70 mA h (800 mA h g−1). To the best of our knowledge, it is the first time to incorporate the GeO2 nanoparticles into the pouch-type cell fabrication process. A single battery is shown to power LED array over 120 bulbs which correspond to driving current 660 mA, a discharge rate 3C–4C, (Fig. 7(e)).
Footnote |
† Electronic supplementary information (ESI) available: Cycling life curve for GeO2 nanoparticles at different electrolyte systems at 0.1C and 1C. See DOI: 10.1039/c6ra20171g |
This journal is © The Royal Society of Chemistry 2016 |