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The electrochemical properties of melt-spun Al–Si–Cu alloys
Materials Chemistry and Physics 129 (2011) 1006–1010
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The electrochemical properties of melt-spun Al–Si–Cu alloys Linping Zhang, Fei Wang, Pu Liang, Xianlei Song, Qing Hu, Zhanbo Sun ∗ , Xiaoping Song, Sen Yang, Liqun Wang MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Received in revised form 26 April 2011 Accepted 19 May 2011 Keywords: Al–Si–Cu alloys Melt spinning Lithium-ion batteries Electrochemical properties
a b s t r a c t Melt spinning was used to prepare Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) alloy anode materials for lithiumion batteries. A metastable supersaturated solid solution of Si and Cu in fcc-Al, ␣-Si and Al2 Cu co-existed in the alloys. Nano-scaled ␣-Al grains, as the matrix, formed in the as-quenched ribbons. The Al74 Si25 Cu1 and Al71 Si25 Cu4 anodes exhibited initial discharge specific capacities of 1539 mAh g−1 , 1324 mAh g−1 and reversible capacities above 472 mAh g−1 , 508 mAh g−1 at the 20th cycle, respectively. The specific capacities reduced as the increase of the Cu content. AlLi intermetallic compound was detected in the lithiated alloys. It is concluded that the lithiation mechanism of the Al–Si-based alloys can be affected by the third component. The structural evolution and volume variation can be mitigated due to the formation of non-equilibrium state and the co-existence of nano-scaled ␣-Al, ␣-Si, and Al2 Cu for the present alloys. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries are important energy storage devices due to their low pollution, high energy density, high output voltage, and long life, etc. Anode materials have a big influence on the properties of lithium-ion batteries. Generally, graphite is commercially used as anode materials, but its low theoretical specific capacity (372 mAh g−1 ) is not able to meet people’s needs for high capacity batteries [1]. In order to develop lithium-ion batteries with a high capacity, some active metals (Sn, Sb, Al, etc.), semimetal (Si) and their alloys have been extensively investigated [2–5]. These materials exhibit high theoretical capacity but poor cycliability. It is widely believed that the cracking, pulverization and failure of anodes caused by the serious structure evolution lead to the poor cycliability [6]. Furthermore, the formation of intermetallic compounds with Li on the surface inhibits the diffusion of Li into a deeper layer, and thus bulk materials are difficult to achieve their theoretical capacity [7–9]. Therefore, nanomaterials are investigated extensively due to their high specific surface areas and the short Li diffusion distance. Recently, Cui et al. [10] reported that core–shell nanowires had high charge storage capacity (1000 mAh g−1 , 3 times of carbon) with 90% capacity retention over 100 cycles. Yu et al. [11] reported that the Ag-coated 3D macroporous Si composite electrode manifested a reversible capacity of 2500 mAh g−1 over 100 cycles. Ruffo et al. [12] developed a silicon
∗ Corresponding author. Tel.: +86 29 8266595; fax: +86 29 83237910. E-mail addresses: [email protected], [email protected] (Z. Sun). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.086
nano-wires which could deliver 2000 mAh g−1 after 80 cycles with good efficiency. However, they are difficult to be produced in large scale. As an effective alternative strategy, composite materials prepared by special methods can partially alleviate the pulverization problem and improve the structural stability. For example, Lei [13] reported that the initial reversible capacities of Al-1 wt% Co3 O4 , Al3 wt% Co3 O4 and Al-5 wt% Co3 O4 were 731 mAh g−1 , 697 mAh g−1 , 656 mAh g−1 and their capacity retention were 66.8, 69.9 and 70.1% after ten cycles, respectively. Doh [14] reported that a composite material comprising Fe, Cu and Si in conjunction with graphite had been obtained through high energy ball milling (HEBM) technique, which could exhibit a high initial discharge of 809 mAh g−1 , with a sustainable reversible capacity >385 mAh g−1 at 30th cycle and high coulomb efficiency >95% after 4th cycle. Chen et al. [15] reported that Al71 Fe9 C20 can deliver the reversible capacity of 436 mAh g−1 at first cycle and 255 mAh g−1 at 15th cycle. Melt spinning is an effective method to prepare non-equilibrium alloys and control their microstructures [16–18]. In our prior investigations [19,20], the melt-spun Al–Si–Mn ribbons prepared as lithium-ion battery anode materials exhibited an improved property. For example, nanocrystalline Al67 Si30 Mn3 had a first discharge capacity of 689 mAh g−1 and maintained 326 mAh g−1 after 40 cycles. It was predicted that Li atoms would be stored in the Albased supersaturated solid solution, phase interfaces and grain boundaries, respectively. Latterly, it was confirmed that the nonequilibrium state of the Al–Si–Mn alloys was a key factor to exhibit favorable electrochemical properties [21]. According to binary alloy phase diagrams, Li and Mn cannot form any intermetallic compound and the solubility of Li in Mn is
L. Zhang et al. / Materials Chemistry and Physics 129 (2011) 1006–1010
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minimum [22], so Mn is a completely inert element for lithiation. No intermetallic compound forms between Li and Cu, but the solubility of Li in Cu is nearly to 20% [23]. Although Cu is also treated as an inert element because a reaction between Li and Cu does not occur at the room temperature, thermodynamic interrelations of Li–Cu and Li–Mn are different, which may affect the electrochemical properties. In this experiment, non-equilibrium Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) anodes were prepared by melt-spinning in order to investigate the dependence of the third component on the electrochemical properties of the Al–Si-based anode materials and develop new anodes materials with improved properties for lithium-ion batteries. The experimental results show that the Al71 Si25 Cu4 anode has high specific capacity and favorable cyclability, and that the lithiation mechanism is different from that of the melt-spun Al–Si–Mn anode [19,20].
2. Experiment Pure Al (99.95 wt%), pure Si (99.99 wt%) and pure Cu (99.95 wt%) were used to prepare the Al–Si–Cu alloys. The starting materials were arc-melted employing a non-consumable tungsten electrode. The ingots were inserted into a quartz tube with a bottom hole and melted by high frequency induction. After they were overheated to a required temperature, the alloy ribbons were then prepared by single roller melt spinning under a pressure of 0.5 atm Ar gas. The ribbons obtained were 3 mm wide and 20–30 m thick. Some ribbons were annealed at 750 K for 2 h. The crystalline structures of the as-quenched and annealed ribbons were measured by a Rigaku D/max 2400 X-ray diffractometer (XRD). The states of the as-quenched and annealed ribbons were analyzed by a NETZSCH STA-449 C Simultaneous Thermal Analyzer. The microstructures of the as-quenched ribbons were observed by a JEM2100 transmission electron microscope (TEM), and the foil specimens used for TEM were prepared by an ion polishing. The as-quenched ribbons were prepared into powders by ball milling for 2 h under Ar gas protection, and then carbon black was added into the powders and mixed homogenously. Subsequently, the mixture was blended with polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP). The ratio of the powder, carbon black and PVDF was 84:8:8. The slurry was evenly coated on the copper foil and dried at 373 K in a vacuum oven for 10 h to remove NMP. Simulation cells were assembled in a dry glove box filled with pure Ar. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC)-diethyl carbonate (DEC) (1:1, v/v). The counter electrode was pure lithium. The electrochemical properties of electrodes were measured by an Arbin BT-2000 Instrument under a constant current density of 0.10 mA cm−2 . In order to activate the electrode materials step by step, the low cut-off voltages were set to 0.13 V and 0.10 V in the first 2 cycles, respectively. The low cut-off voltage of 0.03 V was used in the following cycles. The cyclic voltammetric measurements were measured by a VersaSTAT MC instrument at a scanning speed of 0.1 mV s−1 between 0 V and initial voltage. The morphology of the anodes after 20 cycles was observed by a JSM-7000F Scanning Electron Microscope (SEM), and the crystalline structures of the alloys inserted by Li were measured by the X-ray diffractometer mentioned above.
Fig. 1. X-ray diffraction patterns of melt-spun Al75−X Si25 CuX (X = 1, 4, 7, 10) ribbons.
3. Result Fig. 1 shows the X-ray diffraction (XRD) patterns of the asquenched Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) ribbons. For the Al74 Si25 Cu1 ribbons, the strong diffraction peaks of ␣-Al and ␣Si appeared and the peaks of Al2 Cu were very weak. ␣-Al, ␣-Si and Al2 Cu could be measured clearly when the Cu content was 4%, 7% and 10%, and the peaks of Al2 Cu became stronger as the increase of the Cu content, indicating a larger fraction. The typical Differential Scanning Calorimetry (DSC) curves of the as-quenched and annealed alloys and a XRD pattern of the annealed alloys are shown in Fig. 2. A clear exothermal peak between 400 K and 600 K, as shown in Fig. 2(a), could be detected for the as-quenched alloys, but it disappeared for the annealed ribbons. For the annealed ribbons, the diffraction peaks of ␣-Si and Al2 Cu became much stronger, seen in Fig. 2(b). The results indicate that the metastable supersaturated solid solution of Si and Cu in fcc-Al has been obtained by melt-spinning. It is worthy to be noticed that the formation of Al2 Cu and ␣-Si in an approximate temperature range due to the decomposing of the metastable supersaturated solid solution, as shown in Fig. 2(a). This means the temperature range of the formation Al-based intermatallic compound was lower, compared with
Fig. 2. Differential scanning calorimetry curves of as-quenched and annealed ribbons (a) and X-ray diffraction pattern of the annealed ribbons (b) for the Al71 Si25 Cu4 .
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Fig. 4. Cycle performance of melt-spun Al75−X Si25 CuX (X = 1, 4, 7, 10) anodes.
the first cycle, then declined to 731 mAh g−1 over 3 cycles. Subsequently, a stable cycle performance could be obtained. When X = 4, 7, 10, the anodes illustrated a similar capacity variation tendency that the initial irreversible discharge capacity was large and the capacity fade was slow in the following cycles in the present work. Meanwhile, the capacity reduced as the increase of the Cu content. After 18 cycles, the capacity of the melt-spun Al71 Si25 Cu4 maintained the highest capacity. The XRD patterns of the lithiated Al75−X Si25 CuX anodes are shown in Fig. 5. The peaks of the AlLi intermetallic compound were clearly detected, while those of the Li–Si compound were not found in the lithiated anodes. The ␣-Al, ␣-Si and Al2 Cu which appeared in the as-quenched ribbons were still measured, and no enough evidence indicated that the Al2 Cu compound was alloyed by Li. Fig. 6 displays the cyclic voltammetric curves of the Al75−X Si25 CuX anodes. The CV characteristics were similar to that of pure Al [24] and not dependent on the Cu content in the present work, indicating consistent lithiation mechanism. The cathodic current peak which appeared at about 0.55 V in the first cycle was due to a formation of the solid electrolyte interphase (SEI) [25], which may lead to the large initial irreversible reaction. The anodic current peaks appeared at near 0.65 V in all cycles due to the decomposition of AlLi while the cathodic peaks appeared at 0 V due to the formation. The current peaks corresponding to the lithiation of ␣-Si have
Fig. 3. Microstructures of as-quenched Al74 Si25 Cu1 (a), Al71 Si25 Cu4 (b) and Al68 Si25 Cu7 (c) ribbons. The arrow A, B and C denote ␣-Al, ␣-Si and Al2 Cu, respectively.
that of melt-spun Al–Si–Mn alloys, in which Al-based intermetallic compound formed above 573 K [21]. The matrix of the melt-spun alloys in the present experiment is ␣-Al. Most ␣-Al grains in the ribbons were in nano-scaled but the grains of the Al71 Si25 Cu4 alloy were the finest in the present experiment. It is noticed that ␣-Al, ␣-Si and Al2 Cu co-existed in the alloys, that ␣-Si and ␣-Al spread alternately, and that some Al2 Cu spread between ␣-Al grains when the Cu content was 4%, 7%, and 10%. The TEM images are illustrated in Fig. 3. While the Cu content was more than 7%, ␣-Si became much larger, shown in Fig. 3(c). Fig. 4 illustrates the cycle performance of the melt-spun Al–Si–Cu anodes. The Al74 Si25 Cu1 anodes exhibited a large initial discharge specific capacity of 1539 mAh g−1 , but the capacity declined continuously, resulting in a low retention of 31% after 20 cycles. The capacity of Al71 Si25 Cu4 could reach 1324 mAh g−1 at
Fig. 5. X-ray diffraction patterns of lithiated Al75−X Si25 CuX (X = 1, 4, 7, 10) anodes.
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Fig. 6. Cyclic voltammetric curves for Al74 Si25 Cu1 (a), Al71 Si25 Cu4 (b), Al68 Si25 Cu7 (c) and Al65 Si25 Cu10 (d) anodes.
not been illustrated in the curves [26]. The results are consistent with the XRD patterns of the lithiated alloys (Fig. 5). Fig. 7 shows the SEM images of the Al74 Si25 Cu1 , Al71 Si25 Cu4 and Al68 Si25 Cu7 anodes over 20 cycles. Some cracks appeared due to a volume expansion. The cracks of the Al74 Si25 Cu1 anode were the most serious and a powder exfoliation could be observed. 4. Discussion The melt-spun Al71 Si25 Cu4 anode can exhibit high capacity and favorable cycle performance similar to the Al-based thin films [27], and be better than that of the melt-spun Al–Si–Mn system with 30% Si [20] and many Al-based composite materials synthesized by other methods [13–15], which means that nonequilibrium Al–Si–Cu alloy with optimum composition could be a choice of lithium battery anode material. Therefore, a reasonable design of Al-based anode materials prepared by melt-spinning could improve the properties effectively. Li–Si compounds did not form after lithiation (Fig. 5). It has been proved again that ␣-Si embedded in ␣-Al matrix will not react with Li and the Si atoms in the Al-based supersaturated solid solution will not form a Li–Si phase either during the discharge/charge processes, which is the same with the Al–Si–Mn system [19,20]. Though Al2 Cu anodes can be alloyed by Li [28], the direct evidence that Al2 Cu has not been alloyed by Li in the present work could be inferred from the results of Figs. 5 and 6. The significant volume
expansion that caused by Li atoms inserting into ␣-Si and Al2 Cu embedded in the Al-based supersaturated solid solution results in large elastic strain energy, which will restrict the alloying of ␣-Si and Al2 Cu with Li. There is no doubt that some Li atoms have been stored in the supersaturated Al-based solid solution according to our prior work [19,20]. After lithiation, AlLi intermetallic compound formed, and the relative intensity of ␣-Al in the XRD patterns showed a decreasing trend (Fig. 5), which means that a portion of ␣-Al has formed AlLi. Such a result is different from that of our previous investigations [19,20]. A conclusion could be drawn that the lithiation mechanisms of Al–Si-based alloys are dependent on the third component. For structure evolution, the atom diffusion is necessary. According to the principle of material science, the atom diffusion ability depends strongly on the alloy systems and interactions among components. The DSC curve (Fig. 2) indicated that the formation temperature of Al2 Cu is lower than that of Al/SiMn compound. It could be deduced that the diffusion ability of Al in the melt-spun Al–Si–Cu system is much stronger than that in Al–Si–Mn due to the lower diffusion activation energy, resulting in a formation of AlLi. The supersaturated solid solution of Cu and Si in fcc-Al was obtained by melt spinning, which satisfies the thermodynamic condition for the co-existence of two phases for the lithiated process of the ␣-Al. The appearance of nano-scaled grains can provide a great deal of rapid diffusion channels of Li atoms, which enhances
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work (Fig. 4). If all the Al atoms form AlLi in the Al71 Si25 Cu4 anode, the calculated capacity will be 589 mAh g−1 , which is less than the measured capacity in the first 13 cycles. For the melt-spun Al–Si–Cu ribbons in the present work, the nano-scaled grains created plenty of interfaces. It could be deduced that a lot of Li atoms could be stored in grain boundaries, phase interfaces, dislocations and vacations existing in the Al-based supersaturated solid solution, which is helpful to enhance discharge capacity and improve cycle performance. The Al71 Si25 Cu4 alloy has the finest grains and the best property. 5. Conclusions The melt-spun Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) alloys exhibited high discharge capacity and favorable cycle property. The discharge capacity of more than 508 mAh g−1 could be kept over 20 cycles for the Al71 Si25 Cu4 anode. The AlLi intermetallic compound formed in the lithiated Al75−X Si25 CuX anode while ␣-Si and Al2 Cu did not react with Li evidently. It is concluded that the lithiation mechanism is dependent on the third component for the melt-spun Al–Si-based systems, that the volume variation could be alleviated due to the co-existence of Al2 Cu, ␣-Si and ␣-Al in nano-scale, and that the serious structural evolution could be mitigated due to the existence of non-equilibrium materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (50871081, 51002117 and 51071117) and the National Basic Research Program of China (2010CB635101). References
Fig. 7. Morphologies of lithiated Al74 Si25 Cu1 (a), Al71 Si25 Cu4 (b) and Al68 Si25 Cu7 (c) anodes over 20 cycles.
the diffusion ability of atoms and promote Li atoms to diffuse to a deeper layer. AlLi compound would form in the internal ␣-Al grains of alloy powders, which follows an appearance of stress field caused by volume explanation. In turn, the stress field could limit the further formation of AlLi. As a result, the more serious structure evolution would not occur. On the other side, the existence of Al2 Cu that spreads in the nano-scaled ␣-Al grain boundaries can further enhance the stress field and mitigate the volume variation of cycles caused by ␣-Al alloyed with Li. These factors act simultaneously, resulting in the improved cyclability. The content of Al2 Cu compound in the Al74 Si25 Cu1 ribbons is much less, which is an important reason why Al74 Si25 Cu1 performs poor cycle performance though its initial discharge capacity is the highest (Fig. 4). However, the capacity of Al67 Si25 Cu7 and Al65 Si25 Cu10 will decrease while the volume fractions of Al2 Cu are excess, because the intermetallic compound could not alloy with Li in the present
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The electrochemical properties of melt-spun Al–Si–Cu alloys Linping Zhang, Fei Wang, Pu Liang, Xianlei Song, Qing Hu, Zhanbo Sun ∗ , Xiaoping Song, Sen Yang, Liqun Wang MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
a r t i c l e
i n f o
Article history: Received 3 November 2010 Received in revised form 26 April 2011 Accepted 19 May 2011 Keywords: Al–Si–Cu alloys Melt spinning Lithium-ion batteries Electrochemical properties
a b s t r a c t Melt spinning was used to prepare Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) alloy anode materials for lithiumion batteries. A metastable supersaturated solid solution of Si and Cu in fcc-Al, ␣-Si and Al2 Cu co-existed in the alloys. Nano-scaled ␣-Al grains, as the matrix, formed in the as-quenched ribbons. The Al74 Si25 Cu1 and Al71 Si25 Cu4 anodes exhibited initial discharge specific capacities of 1539 mAh g−1 , 1324 mAh g−1 and reversible capacities above 472 mAh g−1 , 508 mAh g−1 at the 20th cycle, respectively. The specific capacities reduced as the increase of the Cu content. AlLi intermetallic compound was detected in the lithiated alloys. It is concluded that the lithiation mechanism of the Al–Si-based alloys can be affected by the third component. The structural evolution and volume variation can be mitigated due to the formation of non-equilibrium state and the co-existence of nano-scaled ␣-Al, ␣-Si, and Al2 Cu for the present alloys. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries are important energy storage devices due to their low pollution, high energy density, high output voltage, and long life, etc. Anode materials have a big influence on the properties of lithium-ion batteries. Generally, graphite is commercially used as anode materials, but its low theoretical specific capacity (372 mAh g−1 ) is not able to meet people’s needs for high capacity batteries [1]. In order to develop lithium-ion batteries with a high capacity, some active metals (Sn, Sb, Al, etc.), semimetal (Si) and their alloys have been extensively investigated [2–5]. These materials exhibit high theoretical capacity but poor cycliability. It is widely believed that the cracking, pulverization and failure of anodes caused by the serious structure evolution lead to the poor cycliability [6]. Furthermore, the formation of intermetallic compounds with Li on the surface inhibits the diffusion of Li into a deeper layer, and thus bulk materials are difficult to achieve their theoretical capacity [7–9]. Therefore, nanomaterials are investigated extensively due to their high specific surface areas and the short Li diffusion distance. Recently, Cui et al. [10] reported that core–shell nanowires had high charge storage capacity (1000 mAh g−1 , 3 times of carbon) with 90% capacity retention over 100 cycles. Yu et al. [11] reported that the Ag-coated 3D macroporous Si composite electrode manifested a reversible capacity of 2500 mAh g−1 over 100 cycles. Ruffo et al. [12] developed a silicon
∗ Corresponding author. Tel.: +86 29 8266595; fax: +86 29 83237910. E-mail addresses: [email protected], [email protected] (Z. Sun). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.086
nano-wires which could deliver 2000 mAh g−1 after 80 cycles with good efficiency. However, they are difficult to be produced in large scale. As an effective alternative strategy, composite materials prepared by special methods can partially alleviate the pulverization problem and improve the structural stability. For example, Lei [13] reported that the initial reversible capacities of Al-1 wt% Co3 O4 , Al3 wt% Co3 O4 and Al-5 wt% Co3 O4 were 731 mAh g−1 , 697 mAh g−1 , 656 mAh g−1 and their capacity retention were 66.8, 69.9 and 70.1% after ten cycles, respectively. Doh [14] reported that a composite material comprising Fe, Cu and Si in conjunction with graphite had been obtained through high energy ball milling (HEBM) technique, which could exhibit a high initial discharge of 809 mAh g−1 , with a sustainable reversible capacity >385 mAh g−1 at 30th cycle and high coulomb efficiency >95% after 4th cycle. Chen et al. [15] reported that Al71 Fe9 C20 can deliver the reversible capacity of 436 mAh g−1 at first cycle and 255 mAh g−1 at 15th cycle. Melt spinning is an effective method to prepare non-equilibrium alloys and control their microstructures [16–18]. In our prior investigations [19,20], the melt-spun Al–Si–Mn ribbons prepared as lithium-ion battery anode materials exhibited an improved property. For example, nanocrystalline Al67 Si30 Mn3 had a first discharge capacity of 689 mAh g−1 and maintained 326 mAh g−1 after 40 cycles. It was predicted that Li atoms would be stored in the Albased supersaturated solid solution, phase interfaces and grain boundaries, respectively. Latterly, it was confirmed that the nonequilibrium state of the Al–Si–Mn alloys was a key factor to exhibit favorable electrochemical properties [21]. According to binary alloy phase diagrams, Li and Mn cannot form any intermetallic compound and the solubility of Li in Mn is
L. Zhang et al. / Materials Chemistry and Physics 129 (2011) 1006–1010
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minimum [22], so Mn is a completely inert element for lithiation. No intermetallic compound forms between Li and Cu, but the solubility of Li in Cu is nearly to 20% [23]. Although Cu is also treated as an inert element because a reaction between Li and Cu does not occur at the room temperature, thermodynamic interrelations of Li–Cu and Li–Mn are different, which may affect the electrochemical properties. In this experiment, non-equilibrium Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) anodes were prepared by melt-spinning in order to investigate the dependence of the third component on the electrochemical properties of the Al–Si-based anode materials and develop new anodes materials with improved properties for lithium-ion batteries. The experimental results show that the Al71 Si25 Cu4 anode has high specific capacity and favorable cyclability, and that the lithiation mechanism is different from that of the melt-spun Al–Si–Mn anode [19,20].
2. Experiment Pure Al (99.95 wt%), pure Si (99.99 wt%) and pure Cu (99.95 wt%) were used to prepare the Al–Si–Cu alloys. The starting materials were arc-melted employing a non-consumable tungsten electrode. The ingots were inserted into a quartz tube with a bottom hole and melted by high frequency induction. After they were overheated to a required temperature, the alloy ribbons were then prepared by single roller melt spinning under a pressure of 0.5 atm Ar gas. The ribbons obtained were 3 mm wide and 20–30 m thick. Some ribbons were annealed at 750 K for 2 h. The crystalline structures of the as-quenched and annealed ribbons were measured by a Rigaku D/max 2400 X-ray diffractometer (XRD). The states of the as-quenched and annealed ribbons were analyzed by a NETZSCH STA-449 C Simultaneous Thermal Analyzer. The microstructures of the as-quenched ribbons were observed by a JEM2100 transmission electron microscope (TEM), and the foil specimens used for TEM were prepared by an ion polishing. The as-quenched ribbons were prepared into powders by ball milling for 2 h under Ar gas protection, and then carbon black was added into the powders and mixed homogenously. Subsequently, the mixture was blended with polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP). The ratio of the powder, carbon black and PVDF was 84:8:8. The slurry was evenly coated on the copper foil and dried at 373 K in a vacuum oven for 10 h to remove NMP. Simulation cells were assembled in a dry glove box filled with pure Ar. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC)-diethyl carbonate (DEC) (1:1, v/v). The counter electrode was pure lithium. The electrochemical properties of electrodes were measured by an Arbin BT-2000 Instrument under a constant current density of 0.10 mA cm−2 . In order to activate the electrode materials step by step, the low cut-off voltages were set to 0.13 V and 0.10 V in the first 2 cycles, respectively. The low cut-off voltage of 0.03 V was used in the following cycles. The cyclic voltammetric measurements were measured by a VersaSTAT MC instrument at a scanning speed of 0.1 mV s−1 between 0 V and initial voltage. The morphology of the anodes after 20 cycles was observed by a JSM-7000F Scanning Electron Microscope (SEM), and the crystalline structures of the alloys inserted by Li were measured by the X-ray diffractometer mentioned above.
Fig. 1. X-ray diffraction patterns of melt-spun Al75−X Si25 CuX (X = 1, 4, 7, 10) ribbons.
3. Result Fig. 1 shows the X-ray diffraction (XRD) patterns of the asquenched Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) ribbons. For the Al74 Si25 Cu1 ribbons, the strong diffraction peaks of ␣-Al and ␣Si appeared and the peaks of Al2 Cu were very weak. ␣-Al, ␣-Si and Al2 Cu could be measured clearly when the Cu content was 4%, 7% and 10%, and the peaks of Al2 Cu became stronger as the increase of the Cu content, indicating a larger fraction. The typical Differential Scanning Calorimetry (DSC) curves of the as-quenched and annealed alloys and a XRD pattern of the annealed alloys are shown in Fig. 2. A clear exothermal peak between 400 K and 600 K, as shown in Fig. 2(a), could be detected for the as-quenched alloys, but it disappeared for the annealed ribbons. For the annealed ribbons, the diffraction peaks of ␣-Si and Al2 Cu became much stronger, seen in Fig. 2(b). The results indicate that the metastable supersaturated solid solution of Si and Cu in fcc-Al has been obtained by melt-spinning. It is worthy to be noticed that the formation of Al2 Cu and ␣-Si in an approximate temperature range due to the decomposing of the metastable supersaturated solid solution, as shown in Fig. 2(a). This means the temperature range of the formation Al-based intermatallic compound was lower, compared with
Fig. 2. Differential scanning calorimetry curves of as-quenched and annealed ribbons (a) and X-ray diffraction pattern of the annealed ribbons (b) for the Al71 Si25 Cu4 .
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Fig. 4. Cycle performance of melt-spun Al75−X Si25 CuX (X = 1, 4, 7, 10) anodes.
the first cycle, then declined to 731 mAh g−1 over 3 cycles. Subsequently, a stable cycle performance could be obtained. When X = 4, 7, 10, the anodes illustrated a similar capacity variation tendency that the initial irreversible discharge capacity was large and the capacity fade was slow in the following cycles in the present work. Meanwhile, the capacity reduced as the increase of the Cu content. After 18 cycles, the capacity of the melt-spun Al71 Si25 Cu4 maintained the highest capacity. The XRD patterns of the lithiated Al75−X Si25 CuX anodes are shown in Fig. 5. The peaks of the AlLi intermetallic compound were clearly detected, while those of the Li–Si compound were not found in the lithiated anodes. The ␣-Al, ␣-Si and Al2 Cu which appeared in the as-quenched ribbons were still measured, and no enough evidence indicated that the Al2 Cu compound was alloyed by Li. Fig. 6 displays the cyclic voltammetric curves of the Al75−X Si25 CuX anodes. The CV characteristics were similar to that of pure Al [24] and not dependent on the Cu content in the present work, indicating consistent lithiation mechanism. The cathodic current peak which appeared at about 0.55 V in the first cycle was due to a formation of the solid electrolyte interphase (SEI) [25], which may lead to the large initial irreversible reaction. The anodic current peaks appeared at near 0.65 V in all cycles due to the decomposition of AlLi while the cathodic peaks appeared at 0 V due to the formation. The current peaks corresponding to the lithiation of ␣-Si have
Fig. 3. Microstructures of as-quenched Al74 Si25 Cu1 (a), Al71 Si25 Cu4 (b) and Al68 Si25 Cu7 (c) ribbons. The arrow A, B and C denote ␣-Al, ␣-Si and Al2 Cu, respectively.
that of melt-spun Al–Si–Mn alloys, in which Al-based intermetallic compound formed above 573 K [21]. The matrix of the melt-spun alloys in the present experiment is ␣-Al. Most ␣-Al grains in the ribbons were in nano-scaled but the grains of the Al71 Si25 Cu4 alloy were the finest in the present experiment. It is noticed that ␣-Al, ␣-Si and Al2 Cu co-existed in the alloys, that ␣-Si and ␣-Al spread alternately, and that some Al2 Cu spread between ␣-Al grains when the Cu content was 4%, 7%, and 10%. The TEM images are illustrated in Fig. 3. While the Cu content was more than 7%, ␣-Si became much larger, shown in Fig. 3(c). Fig. 4 illustrates the cycle performance of the melt-spun Al–Si–Cu anodes. The Al74 Si25 Cu1 anodes exhibited a large initial discharge specific capacity of 1539 mAh g−1 , but the capacity declined continuously, resulting in a low retention of 31% after 20 cycles. The capacity of Al71 Si25 Cu4 could reach 1324 mAh g−1 at
Fig. 5. X-ray diffraction patterns of lithiated Al75−X Si25 CuX (X = 1, 4, 7, 10) anodes.
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Fig. 6. Cyclic voltammetric curves for Al74 Si25 Cu1 (a), Al71 Si25 Cu4 (b), Al68 Si25 Cu7 (c) and Al65 Si25 Cu10 (d) anodes.
not been illustrated in the curves [26]. The results are consistent with the XRD patterns of the lithiated alloys (Fig. 5). Fig. 7 shows the SEM images of the Al74 Si25 Cu1 , Al71 Si25 Cu4 and Al68 Si25 Cu7 anodes over 20 cycles. Some cracks appeared due to a volume expansion. The cracks of the Al74 Si25 Cu1 anode were the most serious and a powder exfoliation could be observed. 4. Discussion The melt-spun Al71 Si25 Cu4 anode can exhibit high capacity and favorable cycle performance similar to the Al-based thin films [27], and be better than that of the melt-spun Al–Si–Mn system with 30% Si [20] and many Al-based composite materials synthesized by other methods [13–15], which means that nonequilibrium Al–Si–Cu alloy with optimum composition could be a choice of lithium battery anode material. Therefore, a reasonable design of Al-based anode materials prepared by melt-spinning could improve the properties effectively. Li–Si compounds did not form after lithiation (Fig. 5). It has been proved again that ␣-Si embedded in ␣-Al matrix will not react with Li and the Si atoms in the Al-based supersaturated solid solution will not form a Li–Si phase either during the discharge/charge processes, which is the same with the Al–Si–Mn system [19,20]. Though Al2 Cu anodes can be alloyed by Li [28], the direct evidence that Al2 Cu has not been alloyed by Li in the present work could be inferred from the results of Figs. 5 and 6. The significant volume
expansion that caused by Li atoms inserting into ␣-Si and Al2 Cu embedded in the Al-based supersaturated solid solution results in large elastic strain energy, which will restrict the alloying of ␣-Si and Al2 Cu with Li. There is no doubt that some Li atoms have been stored in the supersaturated Al-based solid solution according to our prior work [19,20]. After lithiation, AlLi intermetallic compound formed, and the relative intensity of ␣-Al in the XRD patterns showed a decreasing trend (Fig. 5), which means that a portion of ␣-Al has formed AlLi. Such a result is different from that of our previous investigations [19,20]. A conclusion could be drawn that the lithiation mechanisms of Al–Si-based alloys are dependent on the third component. For structure evolution, the atom diffusion is necessary. According to the principle of material science, the atom diffusion ability depends strongly on the alloy systems and interactions among components. The DSC curve (Fig. 2) indicated that the formation temperature of Al2 Cu is lower than that of Al/SiMn compound. It could be deduced that the diffusion ability of Al in the melt-spun Al–Si–Cu system is much stronger than that in Al–Si–Mn due to the lower diffusion activation energy, resulting in a formation of AlLi. The supersaturated solid solution of Cu and Si in fcc-Al was obtained by melt spinning, which satisfies the thermodynamic condition for the co-existence of two phases for the lithiated process of the ␣-Al. The appearance of nano-scaled grains can provide a great deal of rapid diffusion channels of Li atoms, which enhances
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work (Fig. 4). If all the Al atoms form AlLi in the Al71 Si25 Cu4 anode, the calculated capacity will be 589 mAh g−1 , which is less than the measured capacity in the first 13 cycles. For the melt-spun Al–Si–Cu ribbons in the present work, the nano-scaled grains created plenty of interfaces. It could be deduced that a lot of Li atoms could be stored in grain boundaries, phase interfaces, dislocations and vacations existing in the Al-based supersaturated solid solution, which is helpful to enhance discharge capacity and improve cycle performance. The Al71 Si25 Cu4 alloy has the finest grains and the best property. 5. Conclusions The melt-spun Al75−X Si25 CuX (X = 1, 4, 7, 10 mol%) alloys exhibited high discharge capacity and favorable cycle property. The discharge capacity of more than 508 mAh g−1 could be kept over 20 cycles for the Al71 Si25 Cu4 anode. The AlLi intermetallic compound formed in the lithiated Al75−X Si25 CuX anode while ␣-Si and Al2 Cu did not react with Li evidently. It is concluded that the lithiation mechanism is dependent on the third component for the melt-spun Al–Si-based systems, that the volume variation could be alleviated due to the co-existence of Al2 Cu, ␣-Si and ␣-Al in nano-scale, and that the serious structural evolution could be mitigated due to the existence of non-equilibrium materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (50871081, 51002117 and 51071117) and the National Basic Research Program of China (2010CB635101). References
Fig. 7. Morphologies of lithiated Al74 Si25 Cu1 (a), Al71 Si25 Cu4 (b) and Al68 Si25 Cu7 (c) anodes over 20 cycles.
the diffusion ability of atoms and promote Li atoms to diffuse to a deeper layer. AlLi compound would form in the internal ␣-Al grains of alloy powders, which follows an appearance of stress field caused by volume explanation. In turn, the stress field could limit the further formation of AlLi. As a result, the more serious structure evolution would not occur. On the other side, the existence of Al2 Cu that spreads in the nano-scaled ␣-Al grain boundaries can further enhance the stress field and mitigate the volume variation of cycles caused by ␣-Al alloyed with Li. These factors act simultaneously, resulting in the improved cyclability. The content of Al2 Cu compound in the Al74 Si25 Cu1 ribbons is much less, which is an important reason why Al74 Si25 Cu1 performs poor cycle performance though its initial discharge capacity is the highest (Fig. 4). However, the capacity of Al67 Si25 Cu7 and Al65 Si25 Cu10 will decrease while the volume fractions of Al2 Cu are excess, because the intermetallic compound could not alloy with Li in the present
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