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Electrochemical sodium storage in amorphous GeOx powder
Electrochimica Acta 195 (2016) 192–198
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical sodium storage in amorphous GeOx powder Tetsuya Kajita* , Takashi Itoh Frontier Research Institute for Interdisciplinary Sciences, Tohoku University 6-3 Aoba, Aramaki-aza, Aoba-ku Sendai-shi, Miyagi-ken, 980-8579, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 November 2015 Received in revised form 15 February 2016 Accepted 18 February 2016 Available online 23 February 2016
The amorphous GeOx (x < 1) was synthesized and investigated its suitability as a Na-ion battery anode. GeOx was synthesized by oxidizing Zintl-phase NaGe with isopropyl alcohol at room temperature. The amorphous GeOx showed good electrochemical Na storage as an anode for Na-ion batteries, with first reversible capacity of 342 mAh g1, high power of 237 mAh g1 (6 A g1), and cycle performance, retaining a capacity of 216 mAh g1 after 40 cycles. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: Amorphous Germanium Anode Na-ion battery
1. Introduction Li-ion secondary batteries are used in many applications, such as cell phones, bicycles, laptops, and hybrid and fully electric vehicles [1–3]. However, Li resources are limited and will not be sufficient to meet future demand. Na-ion secondary batteries are a promising next generation technology because Na is abundant, and this may decrease the price of batteries [4]. In addition, new component of battery may be used in Na-ion battery because the electrochemical potential of Na/Na+ is higher than that of Li/Li+. Electrode materials for Na-ion batteries have been studied intensively [4–13]. Large numbers of cathode materials for Na-ion battery have been reported [4–11]. Especially, cathode materials in which the Li site in Li-ion battery cathode material is replaced by Na have shown good performance. Amorphous carbon has also been investigated as an anode material [4,12,13]. However, higher reversible capacity and rate capability are required. Si, Ge, Sn, Sb, and Pb have been considered as high-capacity anodes, and their theoretical capacities for use as Na-ion battery anode are 954, 369, 845, 660, and 485 mAh g1, respectively [4,14]. The Si did not reach theoretical capacities, whereas the Ge, Sn, Sb, and Pb alloys showed high capacity [14–20]. However, Ge showed high capacities only in nanowire and thin film [18–20]. In practice, powdered materials are typically required for making battery active materials. In this work, we used amorphous GeOx (x < 1) powder as a new anode material for Na-ion batteries. The amorphous GeOx powder had a high reversible capacity of
* Corresponding author. Tel.:+ +81 22 795 4495; fax: +81 22 795 7810. E-mail address: [email protected] (T. Kajita). http://dx.doi.org/10.1016/j.electacta.2016.02.117 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
342 mAh g1, a high rate capability of 237 mAh g1 (6 A g1), and good cycle performance of 216 mAh g1 after 40 cycles. These properties arise from the amorphous structure, which allows effective electrochemical Na insertion-extraction. 2. Experimental 2.1. Synthesis of powder amorphous GeOx The NaGe precursor was prepared by a solid-state reaction as follows [21]. In an Ar-filled glove box, a stoichiometric mixture of Na pieces (Kishida Chemical) and Ge powder (Koujyundo Kagaku) was sealed in a stainless steel tube. The tube was heated to 700 C for 24 h. The product was pulverized into a powder with grinding in an Ar-filled glove box for ten minutes. The NaGe powder (0.5 g) was oxidized by isopropyl alcohol (200 mL; Wako chemical) during 1 h and then filtered. The powder was washed in distilled water until ph value of the waste fluid was 7, and filtered. The amorphous GeOx powder dried in a vacuum oven at 70 C overnight. Detail of amorphous will be discussed in Section 3. 2.2. Electrode preparation The amorphous GeOx powder as the active material, a carbon powder conductive additive (Ketjen Black, Lion corp.), and polyamic acid (U-Vanish, Ube industry) were mixed in 1-methyl-2-pyrrolidione at a weight ratio of 80:10:10. The slurry was coated onto Cu foil (15 mm) and evaporated at 120 C for 5 min. Discs with a diameter of 10 mm were punched out of the foil and used as electrodes. Cyclodehydration to form the polyimide was performed by heating at 350 C for 30 min in vacuum [22]. The
T. Kajita, T. Itoh / Electrochimica Acta 195 (2016) 192–198
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Fig. 1. (a) XRD patterns of GeOx powder and electrode. (b) XPS Ge 3d spectra of the GeOx electrode. (c) Annular bright field image of cross sections of a GeOx particle. The electron diffraction pattern is displayed in the inset. (d) SEM image of GeOx powder and (e) SEM image of the GeOx electrode.
T. Kajita, T. Itoh / Electrochimica Acta 195 (2016) 192–198
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Fig. 2. (a) CVs of the GeOx electrode at a scan rate of 0.1 mV s1 between 0.005 and 2.5 V. (b) First two cycle charge-discharge curves of the GeOx electrode at a current rate of 0.2 A g1 between 0.005 and 1.5 V. (c) Galvanostatic profiles at 2th, 5th and 10th.
amount of active material on the electrode ranged from 1.0 to 1.2 mg (1.271.57 mg cm2). 2.3. Cell preparation A CR2032 cell was assembled in an Ar-filled glove box as follows. Na metal was used as the counter electrode, and a glass fiber filter (GB-100R, Advantec) was used as a separator [4]. The liquid electrolyte was 1 M NaPF6 in a mixture of ethylene carbonate and diethyl carbonate (3:7 v/v; Kishida Chemical). 2.4. Electrochemical evaluation CV between 0.005 and 2.5 V was performed at a scan rate of 0.1 mV s1. Cycle performance and power capability were
evaluated after an initial sodiation-desodiation that involved two cycles at a current rate of 0.2 A g1 between 0.005 and 1.5 V. Forty cycle tests were conducted at a current rate of 0.2 A g1. All electrochemical evaluation was performed at room temperature. 2.5. Structural Analysis The crystal structure was characterized by X-ray diffraction by using Cu Ka radiation (Ultima, Rigaku). Because the sodiated samples were unstable in air, they were covered with a Capton film to avoid exposure to air during the measurements. The microstructure and chemical state were determined by using field emission SEM (S-4300E, Hitachi), XPS (Theta Probe, Thermo Fisher Scientific) equipped with an Al Ka (1486.86 eV) X-ray radiation source, and TEM (JEM-2100F, JEOL). The chemical composition was determined by ICP-MS (8800, Agilent).
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3. Result & discussion We synthesized amorphous GeOx (x < 1) powder by oxidizing Zintl phase NaGe with isopropyl alcohol and washing the product with distilled water at room temperature in air. Fig. 1a shows the X-ray diffraction (XRD) patterns of the GeOx powder and GeOx electrode. The XRD pattern of the GeOx electrode contains broad peaks at 2u = 26 and 50 from amorphous GeO [23], and sharp peaks at 2u = 43 , 45 , and 51 from the Cu foil. GeOx remained amorphous after the electrode was fabricated. Large back ground in XRD profile of GeOx powder was due to small quantity of measurement sample and grease used in measurement sample. The X-ray photoelectron spectroscopy (XPS) Ge 3d spectrum of the amorphous GeOx electrode shows several oxidation states (Fig. 1b). The three peaks at 32.8, 31.2, and 29.6 eV are assigned as Ge(+4), Ge (+2), and Ge(0), respectively. A transmission electron microscopy (TEM) image of a GeOx particle on the electrode is shown in Fig. 1c. There were no bright spots in the electron diffraction pattern in the inset of Fig. 1c, which also indicates an amorphous structure. The synthesized GeOx powder had a broad particle size range, from several microns to submicron sizes, as shown in the scanning electron microscopy (SEM) image in Fig. 1d. Cracks were observed on the electrode surface because of the cyclodehydration of polyimide in a vacuum (Fig. 1e). Inductively coupled plasma-mass spectrometry (ICP-MS) analysis gave chemical compositions of x = 0.65. The oxygen content in GeOx was lower than in GeO. Fig. 2a shows the first and second cycles of the cyclic voltammogram (CV) with a scan rate of 0.1 mV s1 in the voltage range of 2.5–0.005 V vs Na/Na+. In the first cycle, reduction peaks were observed at 0.4 V and below 0.1 V. The broad peak at 0.4 V was attributed to the formation of a solid electrolyte interface (SEI) and the formation of Na2O, which is a similar reaction to Li insertion in GeO2 [24,25]. The reaction of the NaxGe alloy occurred below 0.1 V, as previously reported for Ge nanowire [18]. In the first oxidation sweep, peaks were observed at 0.7 V and 1.1 V, which were attributed to Na dealloying and the formation of GeOx, respectively [18,24,25]. The second CV cycle showed reduction-oxidation current peaks similar to the first cycle, except for a decrease in current at 0.4 V. Some formation of SEI and Na2O occurred during the second cycle. These results suggest that the amorphous GeOx electrode shows electrochemically reversible alloying-dealloying. Sodiation-desodiation profiles for the first and second cycles at a scan rate of 0.2 A g1 in the voltage range of 1.5–0.005 V are shown in Fig. 2b. A large polarization (0.7 V) during sodiationdesodiation cycles was observed. Although the reason for the cycle hysteresis is not clear, alloy anodes in Li-ion batteries often show large polarizations during the lithiation-delithiation cycle [26]. The volume change of the alloyed anode material during lithiationdelithiation cycles may cause the large polarization. During the first cycle, the electrode exhibited a high reversible capacity of
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342 mAh g1 and a low efficiency of 53%. The first irreversible capacity was attributed to the formation of SEI and Na2O. Equations 1 and 2 are based on the electrochemical lithiationdelithiation of GeO2 [24,25]. GeOx + (1 + 2x)Na+ + (1 + 2x) e ) NaGe + xNa2O (charge) (1)
NaGe + xNa2O ) Ge + xNa2O + Na+ + e (discharge) (2) Comparing the theoretical capacities (Table S2) estimated from the chemical composition analysis (Table S1) and Eq. (1) and Eq. (2) with the measured capacity, a lower first charge (sodiation) capacity and a slightly larger reversible capacity were observed in our electrochemical measurements. The lower charge capacity may be because formed Na2O which blocked sodiation like as with the sodiation of SnO2 [27,28]. Additionally, Na2O formed in the first sodiation may show a little in electrochemical reversibility, similar to Li2O formation and the sodiation of SnO2 [24,27–29]. In the second cycle, a higher coulombic efficiency of 93% was observed. The GeOx electrode showed high capacity and sodiation-desodiation with an average voltage of 0.7 V. Fig. 2c shows Galvanostatic profile of 2th, 5th and 10th cycles. Sodiation potential on the early stage decreased with increasing the number of cycles, whereas desodiation potential on the terminal stage increased with increasing the number of cycles. Additionally, columbic efficiency was lower after 10th cycles. We have considered that the mechanism of these phenomena is very complex issue. However, the pulverization of amorphous GeOx particles and the decomposition of electrolyte were occurred during charge-discharge tests. Especially, we considered that increasing desodiation potential on the terminal stage may cause weaker electric contact between particles and some particles were exfoliated from electrode due to shrinking particle. Fig. 3a shows XRD patterns of the amorphous GeOx electrode before the first sodiation, after the first sodiation, and after desodiation. The electrodes were rinsed in diethylene carbonate in an Ar-filled glove box. Well-defined peaks were not observed in the XRD patterns, indicating that the amorphous structure was retained during the first sodiation-desodiation cycle. Black allow in XRD profile indicates amorphous GeOx. Broad peaks at 2u = 16 and 35 (red arrows) were observed for the electrode after the first sodiation, suggesting that the Ge-Ge distance increase after sodiation. Furthermore, the shift of the broad peaks (blue allow) was not as high after the first desodiation, which indicates that the distances of Ge-Ge decreased after desodiation. These results show that the volume change of the amorphous GeOx during the first charge was larger than that during the first discharge. SEM images of the electrodes before the first sodiation, after the first sodiation, and after the first desodiation are shown in Fig. 3b–d, respectively. The cracks on the electrode surface almost disappeared because of
Fig. 3. (a) XRD patterns of the pristine, charged, and discharged electrode. (b)–(d) SEM images of electrodes during the first charge-discharge cycle between 0.005 and 1.5 V. The insets in (b)–(d) show high resolution SEM images. (b) Pristine electrode. (c) Charged electrode. (d) Discharged electrode.
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Fig. 4. (a) Cycle performance of the amorphous GeOx anode with a current rate of 0.2 A g1 between 0.005 and 1.5 V. The black plot and blue plot indicate discharge capacity and coulombic efficiency, respectively. (b) XRD pattern of discharged electrode after 40 cycles. (c) SEM image of the discharged electrode after 40 cycles. (d) Charge-discharge rate capability of the amorphous GeOx electrode at 0.005–1.5 V, and scan rates of 6, 4, 2, 1, and 0.5 A g1. (e) Discharge rate capability of the amorphous GeOx electrode with a constant charge rate of 0.2 A g1 at 0.005–1.5 V, and scan rates of 6, 4, 2, 1, and 0.5 A g1. (f) The log i (peak current) vs log y (scan rate) plots during discharge from LSV measurements.
the expansion of the GeOx particles after the first sodiation. Although cracks on the electrode surface appeared after desodiation because the amorphous GeOx particles shrank, the number of cracks was smaller than on the pristine electrode. Therefore, the volume change in amorphous GeOx during the first desodiation was smaller than during the first sodiation. This volume change during the first sodiation-desodiation is consistent with the XRD results. The particle of amorphous GeOx appears to expand together with pulverizing in sodiation from inset of Fig. 3c. In desodiation, the particle of GeOx shrank and became small as shown inset of Fig. 3d. This result indicate that the particle of GeOx was significantly pulverized during the first sodiation-desodiation. Fig. 4a shows the cycle performance of the amorphous GeOx electrode at a current rate of 0.2 A g1 (0.5C) with cut-off voltages of 0.005–1.5 V. 0.342 A g1of first reversible capacity was specified as 1-C value. The cell was tested after an initial sodiationdesodiation that involved the first two cycles shown in Fig. 2b. After 40 cycles, the discharge (desodiation) capacity and its retention ratio were 216 mAh g1 and 68.9%, respectively. However, the coulombic efficiency was low (94%) after 40 cycles, which suggests that the electrolyte decomposed1,11. We confirmed that an electrolyte additive improved the cycle performance, resulting in a discharge (desodiation) capacity and retention ratio of 236 mAh g1 and 77.3%, respectively (Fig. S1). Although the amorphous structure was retained after the cycle test (Fig. 4b), the GeOx particles were broken by the large volume change in amorphous GeOx during the sodiation-desodiation, degrading the cycle performance (Fig. 4c). Carbon coating the particles, optimizing the binder, and using a durable electrolyte are expected to improve the cycle performance of the amorphous GeOx electrode. The rate performance of the amorphous GeOx electrode is shown in Fig. 4d and 4e. The initial discharge (desodiation) capacity of the electrode (first two cycles, Fig. 2b) was 318 mAh g1 with a current rate of 0.2 A g1. The discharge capacities of the electrode at rates of 6
(17.5C), 4 (11.7C), 2 (5.8C), 1 (2.9C), and 0.5 (1.4C) A g1 were 18, 38, 85, 134, and 271 mAh g1, respectively. However, the discharge capacities of the electrode at discharge (Na extraction) rates of 6, 4, 2, and 1, and 0.5 A g1 with a constant charge (Na insertion) rate of 0.2 A g1 were 237, 249, 270, 286, and 300 mAh g1, respectively. These results show that the electrode had a high discharge (desodiation) rate capability and slightly low charge (sodiation) rate capability. The low charge rate capability arose from the decrease of the alloying plateau capacity with the increase of the current rate (Fig. S2). The amorphous GeOx electrode showed a high discharge rate capability. We measured the discharge by linear sweep voltammetry (LSV) at scan rates of 0.1, 0.2, 0.5, 0.7, 1.0, 2.0, and 5.0 mV s1 (Fig. S3) to clarify the reason for the high discharge rate capability. The results indicate that the peak current density was not proportional to the square root of the sweep rate because the discharge reaction included both deintercalation and capacitive reactions, as observed for Li-ion batteries [30]. The capacitive reactions occur in the double layer capacitors, whereas the deintercalation reactions occur mainly in the bulk as a battery reaction. Analysis of the contributions of deintercalation and capacitive reactions to the discharge reaction from the log i vs log y plots (Fig. 4f) and Eq. (3) shows that the deintercalation process dominated during the discharge because the b-value was close to 0.5. Therefore, fast Na-ion extraction may occur in amorphous GeOx during discharge. i = ayb (3) Here, i is the peak current, y is the scan rate, and a and b are adjustable parameters, where the intercalation is dominant at b = 0.5, and the capacitive is dominant at b = 1.0. The amorphous GeOx electrode showed a high reversible capacity, good cycle performance, and high rate capability. In contrast, the crystalline Ge and GeO2 electrodes (Fig. S4) exhibited very low electrochemical sodiation-desodiation as shown Fig. 5.
T. Kajita, T. Itoh / Electrochimica Acta 195 (2016) 192–198
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Fig. 5. Electrochemical Na insertion-extraction in the (a) Ge and (b) GeO2 electrodes at a current rate of 0.2 A g1 between 0.005 and 1.5 V vs Na/Na+.
This indicates that the amorphous structure promotes electrochemical sodiation-desodiation, similar to previous suggestion for lithiation in Ge nanowires [18]. Large atomic distance in amorphous structure causes decreasing overvoltage. Therefore, the sodiation-desodiation in amorphous GeOx might be stable reaction, electrochemically.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.02.117. References
4. Conclusion We synthesized amorphous GeOx powder by a soft chemical method, and demonstrated that the amorphous GeOx electrode showed good electrochemical performance as an anode material for Na-ion battery. The amorphous GeOx powder was synthesized by oxidizing NaGe with isopropyl alcohol and washing in distilled water. This material showed a first reversible capacity of 342 mAh g1 with the average voltage (discharge) of 0.7 V, high power capability of 237 mAh g1 (6 A g1), and cycle performance of 216 mAh g1 after 40 cycles. Higher reversible electrochemical Na storage can be obtained with amorphous material relative to crystalline counterpart. Our results indicate that amorphous anode materials are suited for Na-ion batteries, which extends the range of suitable candidate electrode active materials. Our amorphous GeOx powder electrode advances Na-ion battery development.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgements The authors thank T. Miyazaki, S. Takahashi, Y. Oohira, and N. Akao of the Technical Division, School of Engineering, Tohoku University for carrying out TEM, ICP-MS, and XPS measurements.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical sodium storage in amorphous GeOx powder Tetsuya Kajita* , Takashi Itoh Frontier Research Institute for Interdisciplinary Sciences, Tohoku University 6-3 Aoba, Aramaki-aza, Aoba-ku Sendai-shi, Miyagi-ken, 980-8579, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 November 2015 Received in revised form 15 February 2016 Accepted 18 February 2016 Available online 23 February 2016
The amorphous GeOx (x < 1) was synthesized and investigated its suitability as a Na-ion battery anode. GeOx was synthesized by oxidizing Zintl-phase NaGe with isopropyl alcohol at room temperature. The amorphous GeOx showed good electrochemical Na storage as an anode for Na-ion batteries, with first reversible capacity of 342 mAh g1, high power of 237 mAh g1 (6 A g1), and cycle performance, retaining a capacity of 216 mAh g1 after 40 cycles. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: Amorphous Germanium Anode Na-ion battery
1. Introduction Li-ion secondary batteries are used in many applications, such as cell phones, bicycles, laptops, and hybrid and fully electric vehicles [1–3]. However, Li resources are limited and will not be sufficient to meet future demand. Na-ion secondary batteries are a promising next generation technology because Na is abundant, and this may decrease the price of batteries [4]. In addition, new component of battery may be used in Na-ion battery because the electrochemical potential of Na/Na+ is higher than that of Li/Li+. Electrode materials for Na-ion batteries have been studied intensively [4–13]. Large numbers of cathode materials for Na-ion battery have been reported [4–11]. Especially, cathode materials in which the Li site in Li-ion battery cathode material is replaced by Na have shown good performance. Amorphous carbon has also been investigated as an anode material [4,12,13]. However, higher reversible capacity and rate capability are required. Si, Ge, Sn, Sb, and Pb have been considered as high-capacity anodes, and their theoretical capacities for use as Na-ion battery anode are 954, 369, 845, 660, and 485 mAh g1, respectively [4,14]. The Si did not reach theoretical capacities, whereas the Ge, Sn, Sb, and Pb alloys showed high capacity [14–20]. However, Ge showed high capacities only in nanowire and thin film [18–20]. In practice, powdered materials are typically required for making battery active materials. In this work, we used amorphous GeOx (x < 1) powder as a new anode material for Na-ion batteries. The amorphous GeOx powder had a high reversible capacity of
* Corresponding author. Tel.:+ +81 22 795 4495; fax: +81 22 795 7810. E-mail address: [email protected] (T. Kajita). http://dx.doi.org/10.1016/j.electacta.2016.02.117 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
342 mAh g1, a high rate capability of 237 mAh g1 (6 A g1), and good cycle performance of 216 mAh g1 after 40 cycles. These properties arise from the amorphous structure, which allows effective electrochemical Na insertion-extraction. 2. Experimental 2.1. Synthesis of powder amorphous GeOx The NaGe precursor was prepared by a solid-state reaction as follows [21]. In an Ar-filled glove box, a stoichiometric mixture of Na pieces (Kishida Chemical) and Ge powder (Koujyundo Kagaku) was sealed in a stainless steel tube. The tube was heated to 700 C for 24 h. The product was pulverized into a powder with grinding in an Ar-filled glove box for ten minutes. The NaGe powder (0.5 g) was oxidized by isopropyl alcohol (200 mL; Wako chemical) during 1 h and then filtered. The powder was washed in distilled water until ph value of the waste fluid was 7, and filtered. The amorphous GeOx powder dried in a vacuum oven at 70 C overnight. Detail of amorphous will be discussed in Section 3. 2.2. Electrode preparation The amorphous GeOx powder as the active material, a carbon powder conductive additive (Ketjen Black, Lion corp.), and polyamic acid (U-Vanish, Ube industry) were mixed in 1-methyl-2-pyrrolidione at a weight ratio of 80:10:10. The slurry was coated onto Cu foil (15 mm) and evaporated at 120 C for 5 min. Discs with a diameter of 10 mm were punched out of the foil and used as electrodes. Cyclodehydration to form the polyimide was performed by heating at 350 C for 30 min in vacuum [22]. The
T. Kajita, T. Itoh / Electrochimica Acta 195 (2016) 192–198
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Fig. 1. (a) XRD patterns of GeOx powder and electrode. (b) XPS Ge 3d spectra of the GeOx electrode. (c) Annular bright field image of cross sections of a GeOx particle. The electron diffraction pattern is displayed in the inset. (d) SEM image of GeOx powder and (e) SEM image of the GeOx electrode.
T. Kajita, T. Itoh / Electrochimica Acta 195 (2016) 192–198
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Fig. 2. (a) CVs of the GeOx electrode at a scan rate of 0.1 mV s1 between 0.005 and 2.5 V. (b) First two cycle charge-discharge curves of the GeOx electrode at a current rate of 0.2 A g1 between 0.005 and 1.5 V. (c) Galvanostatic profiles at 2th, 5th and 10th.
amount of active material on the electrode ranged from 1.0 to 1.2 mg (1.271.57 mg cm2). 2.3. Cell preparation A CR2032 cell was assembled in an Ar-filled glove box as follows. Na metal was used as the counter electrode, and a glass fiber filter (GB-100R, Advantec) was used as a separator [4]. The liquid electrolyte was 1 M NaPF6 in a mixture of ethylene carbonate and diethyl carbonate (3:7 v/v; Kishida Chemical). 2.4. Electrochemical evaluation CV between 0.005 and 2.5 V was performed at a scan rate of 0.1 mV s1. Cycle performance and power capability were
evaluated after an initial sodiation-desodiation that involved two cycles at a current rate of 0.2 A g1 between 0.005 and 1.5 V. Forty cycle tests were conducted at a current rate of 0.2 A g1. All electrochemical evaluation was performed at room temperature. 2.5. Structural Analysis The crystal structure was characterized by X-ray diffraction by using Cu Ka radiation (Ultima, Rigaku). Because the sodiated samples were unstable in air, they were covered with a Capton film to avoid exposure to air during the measurements. The microstructure and chemical state were determined by using field emission SEM (S-4300E, Hitachi), XPS (Theta Probe, Thermo Fisher Scientific) equipped with an Al Ka (1486.86 eV) X-ray radiation source, and TEM (JEM-2100F, JEOL). The chemical composition was determined by ICP-MS (8800, Agilent).
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3. Result & discussion We synthesized amorphous GeOx (x < 1) powder by oxidizing Zintl phase NaGe with isopropyl alcohol and washing the product with distilled water at room temperature in air. Fig. 1a shows the X-ray diffraction (XRD) patterns of the GeOx powder and GeOx electrode. The XRD pattern of the GeOx electrode contains broad peaks at 2u = 26 and 50 from amorphous GeO [23], and sharp peaks at 2u = 43 , 45 , and 51 from the Cu foil. GeOx remained amorphous after the electrode was fabricated. Large back ground in XRD profile of GeOx powder was due to small quantity of measurement sample and grease used in measurement sample. The X-ray photoelectron spectroscopy (XPS) Ge 3d spectrum of the amorphous GeOx electrode shows several oxidation states (Fig. 1b). The three peaks at 32.8, 31.2, and 29.6 eV are assigned as Ge(+4), Ge (+2), and Ge(0), respectively. A transmission electron microscopy (TEM) image of a GeOx particle on the electrode is shown in Fig. 1c. There were no bright spots in the electron diffraction pattern in the inset of Fig. 1c, which also indicates an amorphous structure. The synthesized GeOx powder had a broad particle size range, from several microns to submicron sizes, as shown in the scanning electron microscopy (SEM) image in Fig. 1d. Cracks were observed on the electrode surface because of the cyclodehydration of polyimide in a vacuum (Fig. 1e). Inductively coupled plasma-mass spectrometry (ICP-MS) analysis gave chemical compositions of x = 0.65. The oxygen content in GeOx was lower than in GeO. Fig. 2a shows the first and second cycles of the cyclic voltammogram (CV) with a scan rate of 0.1 mV s1 in the voltage range of 2.5–0.005 V vs Na/Na+. In the first cycle, reduction peaks were observed at 0.4 V and below 0.1 V. The broad peak at 0.4 V was attributed to the formation of a solid electrolyte interface (SEI) and the formation of Na2O, which is a similar reaction to Li insertion in GeO2 [24,25]. The reaction of the NaxGe alloy occurred below 0.1 V, as previously reported for Ge nanowire [18]. In the first oxidation sweep, peaks were observed at 0.7 V and 1.1 V, which were attributed to Na dealloying and the formation of GeOx, respectively [18,24,25]. The second CV cycle showed reduction-oxidation current peaks similar to the first cycle, except for a decrease in current at 0.4 V. Some formation of SEI and Na2O occurred during the second cycle. These results suggest that the amorphous GeOx electrode shows electrochemically reversible alloying-dealloying. Sodiation-desodiation profiles for the first and second cycles at a scan rate of 0.2 A g1 in the voltage range of 1.5–0.005 V are shown in Fig. 2b. A large polarization (0.7 V) during sodiationdesodiation cycles was observed. Although the reason for the cycle hysteresis is not clear, alloy anodes in Li-ion batteries often show large polarizations during the lithiation-delithiation cycle [26]. The volume change of the alloyed anode material during lithiationdelithiation cycles may cause the large polarization. During the first cycle, the electrode exhibited a high reversible capacity of
195
342 mAh g1 and a low efficiency of 53%. The first irreversible capacity was attributed to the formation of SEI and Na2O. Equations 1 and 2 are based on the electrochemical lithiationdelithiation of GeO2 [24,25]. GeOx + (1 + 2x)Na+ + (1 + 2x) e ) NaGe + xNa2O (charge) (1)
NaGe + xNa2O ) Ge + xNa2O + Na+ + e (discharge) (2) Comparing the theoretical capacities (Table S2) estimated from the chemical composition analysis (Table S1) and Eq. (1) and Eq. (2) with the measured capacity, a lower first charge (sodiation) capacity and a slightly larger reversible capacity were observed in our electrochemical measurements. The lower charge capacity may be because formed Na2O which blocked sodiation like as with the sodiation of SnO2 [27,28]. Additionally, Na2O formed in the first sodiation may show a little in electrochemical reversibility, similar to Li2O formation and the sodiation of SnO2 [24,27–29]. In the second cycle, a higher coulombic efficiency of 93% was observed. The GeOx electrode showed high capacity and sodiation-desodiation with an average voltage of 0.7 V. Fig. 2c shows Galvanostatic profile of 2th, 5th and 10th cycles. Sodiation potential on the early stage decreased with increasing the number of cycles, whereas desodiation potential on the terminal stage increased with increasing the number of cycles. Additionally, columbic efficiency was lower after 10th cycles. We have considered that the mechanism of these phenomena is very complex issue. However, the pulverization of amorphous GeOx particles and the decomposition of electrolyte were occurred during charge-discharge tests. Especially, we considered that increasing desodiation potential on the terminal stage may cause weaker electric contact between particles and some particles were exfoliated from electrode due to shrinking particle. Fig. 3a shows XRD patterns of the amorphous GeOx electrode before the first sodiation, after the first sodiation, and after desodiation. The electrodes were rinsed in diethylene carbonate in an Ar-filled glove box. Well-defined peaks were not observed in the XRD patterns, indicating that the amorphous structure was retained during the first sodiation-desodiation cycle. Black allow in XRD profile indicates amorphous GeOx. Broad peaks at 2u = 16 and 35 (red arrows) were observed for the electrode after the first sodiation, suggesting that the Ge-Ge distance increase after sodiation. Furthermore, the shift of the broad peaks (blue allow) was not as high after the first desodiation, which indicates that the distances of Ge-Ge decreased after desodiation. These results show that the volume change of the amorphous GeOx during the first charge was larger than that during the first discharge. SEM images of the electrodes before the first sodiation, after the first sodiation, and after the first desodiation are shown in Fig. 3b–d, respectively. The cracks on the electrode surface almost disappeared because of
Fig. 3. (a) XRD patterns of the pristine, charged, and discharged electrode. (b)–(d) SEM images of electrodes during the first charge-discharge cycle between 0.005 and 1.5 V. The insets in (b)–(d) show high resolution SEM images. (b) Pristine electrode. (c) Charged electrode. (d) Discharged electrode.
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Fig. 4. (a) Cycle performance of the amorphous GeOx anode with a current rate of 0.2 A g1 between 0.005 and 1.5 V. The black plot and blue plot indicate discharge capacity and coulombic efficiency, respectively. (b) XRD pattern of discharged electrode after 40 cycles. (c) SEM image of the discharged electrode after 40 cycles. (d) Charge-discharge rate capability of the amorphous GeOx electrode at 0.005–1.5 V, and scan rates of 6, 4, 2, 1, and 0.5 A g1. (e) Discharge rate capability of the amorphous GeOx electrode with a constant charge rate of 0.2 A g1 at 0.005–1.5 V, and scan rates of 6, 4, 2, 1, and 0.5 A g1. (f) The log i (peak current) vs log y (scan rate) plots during discharge from LSV measurements.
the expansion of the GeOx particles after the first sodiation. Although cracks on the electrode surface appeared after desodiation because the amorphous GeOx particles shrank, the number of cracks was smaller than on the pristine electrode. Therefore, the volume change in amorphous GeOx during the first desodiation was smaller than during the first sodiation. This volume change during the first sodiation-desodiation is consistent with the XRD results. The particle of amorphous GeOx appears to expand together with pulverizing in sodiation from inset of Fig. 3c. In desodiation, the particle of GeOx shrank and became small as shown inset of Fig. 3d. This result indicate that the particle of GeOx was significantly pulverized during the first sodiation-desodiation. Fig. 4a shows the cycle performance of the amorphous GeOx electrode at a current rate of 0.2 A g1 (0.5C) with cut-off voltages of 0.005–1.5 V. 0.342 A g1of first reversible capacity was specified as 1-C value. The cell was tested after an initial sodiationdesodiation that involved the first two cycles shown in Fig. 2b. After 40 cycles, the discharge (desodiation) capacity and its retention ratio were 216 mAh g1 and 68.9%, respectively. However, the coulombic efficiency was low (94%) after 40 cycles, which suggests that the electrolyte decomposed1,11. We confirmed that an electrolyte additive improved the cycle performance, resulting in a discharge (desodiation) capacity and retention ratio of 236 mAh g1 and 77.3%, respectively (Fig. S1). Although the amorphous structure was retained after the cycle test (Fig. 4b), the GeOx particles were broken by the large volume change in amorphous GeOx during the sodiation-desodiation, degrading the cycle performance (Fig. 4c). Carbon coating the particles, optimizing the binder, and using a durable electrolyte are expected to improve the cycle performance of the amorphous GeOx electrode. The rate performance of the amorphous GeOx electrode is shown in Fig. 4d and 4e. The initial discharge (desodiation) capacity of the electrode (first two cycles, Fig. 2b) was 318 mAh g1 with a current rate of 0.2 A g1. The discharge capacities of the electrode at rates of 6
(17.5C), 4 (11.7C), 2 (5.8C), 1 (2.9C), and 0.5 (1.4C) A g1 were 18, 38, 85, 134, and 271 mAh g1, respectively. However, the discharge capacities of the electrode at discharge (Na extraction) rates of 6, 4, 2, and 1, and 0.5 A g1 with a constant charge (Na insertion) rate of 0.2 A g1 were 237, 249, 270, 286, and 300 mAh g1, respectively. These results show that the electrode had a high discharge (desodiation) rate capability and slightly low charge (sodiation) rate capability. The low charge rate capability arose from the decrease of the alloying plateau capacity with the increase of the current rate (Fig. S2). The amorphous GeOx electrode showed a high discharge rate capability. We measured the discharge by linear sweep voltammetry (LSV) at scan rates of 0.1, 0.2, 0.5, 0.7, 1.0, 2.0, and 5.0 mV s1 (Fig. S3) to clarify the reason for the high discharge rate capability. The results indicate that the peak current density was not proportional to the square root of the sweep rate because the discharge reaction included both deintercalation and capacitive reactions, as observed for Li-ion batteries [30]. The capacitive reactions occur in the double layer capacitors, whereas the deintercalation reactions occur mainly in the bulk as a battery reaction. Analysis of the contributions of deintercalation and capacitive reactions to the discharge reaction from the log i vs log y plots (Fig. 4f) and Eq. (3) shows that the deintercalation process dominated during the discharge because the b-value was close to 0.5. Therefore, fast Na-ion extraction may occur in amorphous GeOx during discharge. i = ayb (3) Here, i is the peak current, y is the scan rate, and a and b are adjustable parameters, where the intercalation is dominant at b = 0.5, and the capacitive is dominant at b = 1.0. The amorphous GeOx electrode showed a high reversible capacity, good cycle performance, and high rate capability. In contrast, the crystalline Ge and GeO2 electrodes (Fig. S4) exhibited very low electrochemical sodiation-desodiation as shown Fig. 5.
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+
Potential (V vs Na/Na )
a 2 Ge electrode
1
1st
2nd
0
0
+
Potential (V vs Na/Na )
b
100 200 300 -1 Capacity (mAh g )
400
2 GeO2 electrode
1st
1
2nd
0
0
100 200 300 -1 Capacity (mAh g )
400
Fig. 5. Electrochemical Na insertion-extraction in the (a) Ge and (b) GeO2 electrodes at a current rate of 0.2 A g1 between 0.005 and 1.5 V vs Na/Na+.
This indicates that the amorphous structure promotes electrochemical sodiation-desodiation, similar to previous suggestion for lithiation in Ge nanowires [18]. Large atomic distance in amorphous structure causes decreasing overvoltage. Therefore, the sodiation-desodiation in amorphous GeOx might be stable reaction, electrochemically.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.02.117. References
4. Conclusion We synthesized amorphous GeOx powder by a soft chemical method, and demonstrated that the amorphous GeOx electrode showed good electrochemical performance as an anode material for Na-ion battery. The amorphous GeOx powder was synthesized by oxidizing NaGe with isopropyl alcohol and washing in distilled water. This material showed a first reversible capacity of 342 mAh g1 with the average voltage (discharge) of 0.7 V, high power capability of 237 mAh g1 (6 A g1), and cycle performance of 216 mAh g1 after 40 cycles. Higher reversible electrochemical Na storage can be obtained with amorphous material relative to crystalline counterpart. Our results indicate that amorphous anode materials are suited for Na-ion batteries, which extends the range of suitable candidate electrode active materials. Our amorphous GeOx powder electrode advances Na-ion battery development.
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Acknowledgements The authors thank T. Miyazaki, S. Takahashi, Y. Oohira, and N. Akao of the Technical Division, School of Engineering, Tohoku University for carrying out TEM, ICP-MS, and XPS measurements.
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