Mechanochemically Metamorphosed Composites of Homogeneous Nanoscale Silicon and Silicate Oxides with Lithium and Metal Compounds ()
1. Introduction
Si-based Li-ion batteries (LIBs) have great potential because of the high capacity of the Si-based anodes relative to that of graphite anodes [1] . However, a major obstacle to achieve a satisfactory reversible capacity in practical use is the large change in the Si volume observed during charge-discharge operations [2] [3] [4] [5] [6] and the irreversible formation of Li components. Many researchers have attempted to overcome these problems using nanostructures [4] [7] - [14] or other approaches, such as partial oxidization [7] [15] [16] , composite formation with other materials [7] [9] [11] [17] [18] [19] [20] , liquid-phase synthesis, and various battery production methods. Improvements in the stability of the prepared Si anodes have been observed from these attempts, leading to an improvement in LIB performance. We consider that further significant improvements may be obtained from synergistic effects when two or more approaches are simultaneously introduced in a simple operation. Accordingly, we reported the optimization of the cycling properties of an active Si anode by employing a composite of Si with CuO that was formed by a simple ball milling process with a planetary ball mill machine [21] . Here, we propose a mechanochemical approach that involves ball milling with Si, Li2O, and CuO, and we describe the mechanism by which the obtained composite achieves a high charge-discharge performance. We successfully synthesized composites using homogeneous, nanoscale Si grains in a matrix of partially oxidized ground Si (SiO) as an active material. An LIB using the prepared composite showed a high initial coulombic efficiency (ratio between the reversible discharge capacity and the charge capacity) with good charge-discharge characteristics and an optimized cycling performance of over 3000 mAh/g after 800 cycles (Figure 1). The prepared composite of Si, Li2O, and CuO shows improved electrical and ionic conductivities in the LIB system and is resistant to crystal collapse because of the expansion and contraction of Si in the composite. There are many reports of attempts to optimize both the electrical and ionic conductivities [22] [23] [24] [25] [26] ; however, there are no previous attempts to synthesis active materials that include Li atoms to maintain the high performance of an LIB that lacks Li atoms for charge-discharge characteristics. We propose the synthesis of a composite with active Li compounds to compensate for the lack of Li ions in the anode of an LIB system along with Li-Si or other metal-Si alloys to optimize the electrical and ionic conductivities using a simple mechanochemical process.
2. Experimental
2.1. Composite of Si, Li2O, and CuO Particles
A mixture of Si, Li2O, and CuO particles (i.e. Si + Li2O + CuO = 2.0 g) with average particle diameters of 4 μm (Kojundo Chemistry Laboratory Co., Ltd., Japan) was placed in a zirconia mill pot (45 cm3 inner volume) with 7 zirconia balls (15 mm φ in diameter); ball milling was performed using a planetary ball mill machine (Fritsch Pulverisette-7) at approximately 600 rpm without heating and in air to produce aggregates composited with Si, Li2O, and CuO. We then established a new anode composite to exploit the anticipated mechanochemical reaction between Si, Li2O, and CuO when ground together. We designed particles
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Figure 1. (a) Initial charge-discharge properties of the prepared composite with Si, Li2O, and CuO at loading current densities of 0.01 mA/cm2 (0.003 C), 3.2 mA/cm2 (1 C), and 9.6 mA/cm2 (3 C); (b) Comparison of the cycling properties of the anode with the prepared Si, Li2O, and CuO composite (red line); Si and CuO composite (blue line); Si and Li2O composite (green line); Si, Li2O, and CuO mixture (dotted line); and Si particles as a reference (black line). The attenuation observed for the Si, Li2O, and CuO composite after 800 cycles is due to peeling of the film from the Cu foil.
composited with Si (1.630 g), Li2O (0.043 g), and CuO (0.327 g) for ball milling. The operation time for rotation of the ball mills was 3 h. Partial oxidation of Si at a molar ratio of Si:Li2O:CuO of 10:0.25:0.75 endowed the composite with an excellent LIB performance.
2.2. Preparation of the Composite Electrodes
The Si composite electrodes were prepared as follows. The prepared composite was mixed with a binder composed of polyamic acid (Ube-kousan KK Company, Japan) and acetylene black (AB; Denkikagaku, Japan) as a conductive material in a 1-methyl-2-pyrrolidone solution. The Si composite:binder:AB weight ratio was 70:20:10. The slurry of the electrode components was cast onto a Cu foil and dried at 70˚C for 20 min in air. The cast electrodes were cut to 10 mm φ. The obtained electrodes had thicknesses in the range of 40 - 50 μm. The electrodes were further dried at 550˚C under vacuum for 3 h and then pressed at 200 kgf/cm2. The specific capacity was calculated according to the weight of the composite/binder/AB. Electrochemical tests of the composite electrodes were conducted using two-electrode test coin cells (type 2032; Housen, Japan) with a separator, and a gasket was used to hold the electrode in place.
2.3. Assembly of a Test Coin Cell for the LIB
The coin cells were assembled in an Ar-filled glove box using 1 M LiPF6 in a mixed ethylene carbonate, diethylene carbonate, and dimethyl carbonate solution (60:25:15, v/v) as the electrolyte. For high charge-discharge operations, a LiCoO2-coated film (300-μm thick) at a capacity of 12.0 mAh/cm2 was employed as the cathode counter electrode, whereas a metal Li-foil was employed for impedance measurements. The separator was a polyethylene/polypropylene/poly- ethylene multi-stacked film with a thickness of 10 μm. The electrochemical performances of the two-electrode coin cells were evaluated using a potentiostat (Hokuto Denko Co. Ltd., Japan).
3. Results and Discussion
3.1. Electrochemical Properties of the Prepared Composite
Figure 1(a) shows the electrochemical performance of the composite anode evaluated in a 2032-type coin cell with a LiCoO2 cathode in a constant-current charge-discharge test in the voltage range of 1.6 - 4.2 V with current densities of 0.01 mA/cm2 (0.003 C), 3.2 mA/cm2 (1 C), and 9.6 mA/cm2 (3 C) at 27˚C. The state of the charge of the battery was set to 100%. An LIB anode composed of Si, Li2O, and CuO was fabricated on a Cu foil with a molar ratio of 10:0.25:0.75 to obtain good experimental charge-discharge characteristics. The results indicate that the charge and reversible discharge capacities depend on the components of the active material. The prepared composite, shown by the red line, exhibited an initial coulombic efficiency of 108.5% at a current density of 0.01 mA/cm2 (0.003 C), as shown in Figure 1(a). The blue and black lines indicate that the initial coulombic efficiencies were 102.3% and 99.86% at current densities of 3.2 mA/cm2 (1 C) and 9.6 mA/cm2 (3 C), respectively. The average initial coulombic efficiency of the Si active material in the LIB was approximately 80%; however, the prepared composite attained a value of over 99% with a high loading rate. The Li compounds in the prepared composite may optimize the charge-discharge properties. We previously reported a mechanochemically synthesized composite of Si and CuO with a high initial coulombic efficiency of 95.1% [21] . The addition of Li ions is expected to improve the electrical and ionic conductivities of the active material. Moreover, because ceramic CuO and Li2O are known as non-occlusive and non-conductive materials, respectively, they do not function as active materials in the LIB. In this study, the composites, which consist of aggregates of Si, other conductive nanoscale grains, and a Si oxide material produced mechanochemically by ball milling, were successfully employed as anodes to occlude Li ions with a high coulombic efficiency from the first charge-discharge stage.
Figure 1(b) shows the cycling properties in the voltage range of 1.6 - 4.2 V at a current density of 9.6 mA/cm2 (3 C) with LiCoO2 employed as the cathode in a coin cell. The measured anodes employ a composite of Si, Li2O, and CuO produced by a mechanochemical process (red line), a composite of Si and CuO (blue line), a composite with Si and Li2O (yellow line), a mixture of Si, Li2O, and CuO (dotted yellow line), and Si particles as a reference (black line). The Si/Li2O/CuO and Si/CuO composites exhibited reversible capacities above 3000 mAh/g after 800 cycles; notably, the Si/Li2O/CuO composite showed a high reversible capacity at 1000 cycles. This result contrasts with the behaviour of bare Si particles and the mixtures of Si/CuO and Si/Li2O, which showed capacities below 50% after relatively few cycles. The Li2O and CuO ceramics in the mixture do not function as active materials and have low cycling properties when mixed with Si, as shown in Figure 1(b). However, the Si/Li and Si/Cu compounds synthesized using the ball milling process with this mixture increase the electrical and ionic conductivities in the prepared composite, thereby helping to maintain a high capacity over 100 charge-discharge cycles.
3.2. Mechanochemical Composition
Mechanochemical processes have been widely studied [27] [28] [29] . One line of research has focused on mechanochemical reduction by employing one element and one compound [30] [31] , such as the reduction of CuO by co-grinding with Al, Mg, or Si to form Cu powders [32] . However, the products obtained using equimolar CuO and Si fail to achieve high LIB performances because a Si oxide, probably SiO2, synthesized by the mechanochemical process leads to excessive oxide atoms in Si, and the composite does not function as an active material. In contrast, at a molar ratio of 10:0.75, Si was partially oxidized by the oxide atoms in CuO, producing a composite with an excellent LIB performance. In this case, the chemical reaction between Si and CuO is given by Equation (1). Thus, we attempted to establish a new anode composite to exploit the anticipated mechanochemical reaction between Si and CuO upon ball milling.
(1)
Furthermore, the mechanochemical reaction between Si and Li2O when mixed at a molar ratio of 10:0.25 (Si:O) is expected to produce a composite by a non-equilibrium active reaction. Li has a lower electronegativity than Si or Cu formed by a mechanochemical process [32] , does not react to reduce oxide atoms, and is not mechanochemically synthesized.
Figure 2 shows the crystallinity and ratio of components with Li atoms for the composite obtained by the mechanochemical process of Si, Li2O, and CuO. The oxide atoms in CuO are reduced, and residual Cu is expected to form an alloy
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Figure 2. (a) XRD patterns of the Si, Li2O, and CuO composite and each raw material before the ball milling process; (b) XPS spectrum of the prepared composite, enlarged in the region of the Li peak from 50 to 60 eV. The red spectrum can be deconvoluted into peaks corresponding to Li ions with various oxidation numbers. The Li ion peak mainly consists of Li2SiO3 or Li4Si (blue line), with minor LiOH (aqua line) and Li2O (black line) components.
with Si [27] . According to the X-ray diffraction (XRD; Rigaku Co. Ltd., Japan) pattern in Figure 2(a), the Cu-Si alloy is in the form of Cu3Si [33] and acts as a current collector; this alloy is electrically conductive and an important material for charge-discharge characteristics, which is determined by comparing the Si/Li2O/CuO composite with the Si/Li2O composite in Figure 1(b), but does not function as an active material. In the XRD spectrum of the composite, no peaks are observed for CuO and Li2O. Thus, the compounds synthesized from the reduction and oxidation of CuO or Li2O were amorphous in the prepared composite. Si, Li, and the other oxide compounds were examined using high-resolution Si2p X-ray photoelectron spectroscopy (XPS; JEOL, Japan); a spectrum of the synthesized composite is shown in Figure 2(b). A Si-Li oxide or Si-Li alloy is expected to exist in the Si, Li2O, and CuO composite. The enlargement of the XPS peak at ~50 - 60 eV, shown in Figure 2(b), indicates the presence of a variety of Li ions with different oxidation numbers and mainly consists of Li-silicate oxide materials and Li alloys, namely Li2SiO3, Li4Si, LiOH, and Li2O. The content ratio of the different Li components in the composite was calculated by deconvolution of the peak position, which was calibrated using the C1s signal. Li was found mainly in the form of Li2SiO3 or Li4Si throughout the synthesized composite, which confirms the presence of a Li-silicate oxide and Li-Si alloy in the composite, and the reaction outlined in Equation (2) is presumed. Moreover, CuO and Li2O do not react with each other to form other components.
(2)
Li2SiO3 has a higher free energy of formation than Li2O and easily decomposes to a Si-Li oxide or Si-Li alloy; furthermore, Li2SiO3 does not function as an LIB active material.
Figure 3 shows an analysis of the prepared composite using scanning electron microscopy, scanning transmission electron microscopy, and transmission electron microscopy (SEM/STEM/TEM; Hitachi High-Technologies Co. Ltd.,
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Figure 3. STEM images of the Si, Li2O, and CuO composite. (a) SEM image showing the cross-sectional view of the microscale particles obtained by the ball milling process with Si, Li2O, and CuO; (b) STEM cross-sectional view of the composite particle; (c) Enlarged STEM image of the areas marked in (a). The black area inside the white dotted line indicates Cu3Si alloy grains in the composite particle; (d) TEM diffraction image showing the diffraction patterns of Si, Si-Li alloy, and Cu-Si alloy compounds in the STEM measurement area of (c).
Japan). The SEM overview image (Figure 3(a)) reveals the composite particles obtained by the ball milling process with Si, Li2O, and CuO, and the STEM cross-sectional views in Figure 3(b) and Figure 3(c) show the bright field images of an anode electrode film in the areas indicated by red circles in Figure 3(a). The bright grey areas in these images correspond to the composite of Si, Li, and O atoms, whereas the dark grey areas indicate the presence of Cu3Si in the composite [21] . The crystal lattices indicate the orientation of each nanoscale Si or Li4Si, and Cu3Si crystal lattice from the results in Figure 2(a). The high-resolution STEM images in Figure 3(c) confirm that each grain has a random crystal orientation and that the composite mainly comprises polycrystalline Si with nanoscale grains averaging 10 nm φ or Si-Li alloy grains, Cu3Si nanoscale grains, and other compounds including a silicone monoxide and Li-silicate oxide components. Figure 3(d) represents crystallization of the prepared composite, nanoscale Si grains, and Cu3Si grains.
The presence of Li or Cu in the composite is hypothesized to improve the electrical and ionic conductivities of the composite. Thus, impedance measurements were carried out to identify the elements that improve the electrical and ionic conductivities. Figure 4 shows the impedance measurement results for the Si/Li2O/CuO, Si/Li2O, and Si/CuO composites, with that for Si particles as a reference. The samples were evaluated in a 2032-type coin cell with metal Li foil as the cathode. The Si/Li2O/CuO and Si/Li2O composites, which include Li compounds, do not have a capacitance element in the impedance chart in Figure 4(a) in the low resistance range (shaded area), meaning that the ionic resistance element is almost zero when Li ions pass into the composite. Moreover, the impedance elements in Figure 4(b) in the high resistance range (shaded area) show that the composites including Li atoms have lower impedance elements, and the electrical conductance and ionic conductance are better than those of the other
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Figure 4. Comparison of the impedance charts of the composites with Si, Li2O, and CuO (red line), Si and Li2O (yellow line), and Si and CuO (blue line), with Si as a reference (green line). The shaded area of the impedance chart in (a) with a low resistance (<15 Ω) indicates the electrical and ionic conductivities of a Li ion passing into a particle and is modelled as a parallel circuit of capacitance and resistance. The shaded area of the chart in (b) with a high resistance (<300 Ω) indicates the impedance component passing through the border of a particle or from one particle to another.
composites that do not include Li atoms. These results confirm that the presence of Li compounds in the composite serve to improve the mobility of electrons and Li ions within the composite in LIBs.
3.3. Electrochemical Mechanism of the Composite
We observed punctate materials in the composite, leading us to surmise that the composite is a mixture of compounds of Si, Li2O, and CuO. The atomic distributions of Si and Cu were determined using energy dispersive X-ray spectroscopy (EDX; Hitachi High-Technologies Corporation, Japan), and the reference Si atoms and Li atoms were detected using electron energy loss spectroscopy (EELS; Hitachi High-Technologies Corporation, Japan), as shown in Figure 5. Each distribution was measured before the initial charge and after 800 cycles. The EDX and EELS results show that the composite contained Si, Li, and Cu grains before the initial charge from the mechanochemical process. Si, Li, and Cu were homogeneously dispersed in the composite before the initial charge; after 800 cycles, Li atoms were practically non-existent in the composite. Li atoms in the composite could be used to discharge the capacity of the anode during charge-discharge operation to maintain a high capacity. In that case, an irreversible Li-Si alloy was synthesized during the charge-discharge process, and the synthesized Li atoms in the composite were made available to compensate for the capacity attenuation over many cycles. Moreover, a major obstacle to achieving a satisfactory reversible capacity in practical use lies in the large change in the Si volume observed during charge-discharge operation. The shape
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Figure 5. Distributions of Si and Cu by EDX and Si and Li by EELS in the composite with Si, Li2O, and CuO. ((a) and (b)) SEM cross-sectional views of the composite with Si, Li2O, and CuO before the initial charge, EDX distribution maps of Si and Cu, and EELS distribution maps of Si and Li. ((c) and (d)) SEM cross-sectional view of the composite with Si, Li2O, and CuO; EDX and EELS maps after 800 charge-discharge cycles at 3000 mAh/g.
of the composite and atomic distributions of Si, Li, and Cu were maintained during the testing cycles, and no collapse of the structure was observed in the presence of SiO as a buffer matrix. The amorphous structure of SiO [34] [35] [36] [37] in the composite is expected to play a role in mitigating the expansion of active materials with nano-Si grains and including Li ions. In addition, Figure 5(c) and Figure 5(d) confirm that the composite did not change volume after the charge-discharge process, and crystal or nanoscale grain collapse was prevented during the expansion and contraction of Si in the prepared composite. In (d), EELS-Li revealed Li atoms distributed around the composite particle because Li atoms precipitated from the residual electrolyte in the LIB.
The specific gravity of the prepared composite is approximately 2.12 g/cm3, which is less than the specific gravity of Si (2.33 g/cm3). The prepared composite contains a matrix of silicon monoxide as a buffer material to maintain the expansion of the composite when inserting Li ions into Si nanoparticles and other active materials. This could prevent the expansion of Si and optimise the charge-discharge characteristics.
We expect a Li-Si alloy (likely Li4Si) to be present in the composite; Li ion distribution in the prepared composite is demonstrated by the EDX measurements shown in Figure 5. Thus, the Si, Li2O, and CuO composite is confirmed as an aggregate of nanoscale grains based on Si, Cu3Si, and other materials. The structure of the composite is hypothesized in Figure 6 to involve nanoscale Si grains to prevent cracking through numerous repetitions of the charge-discharge cycle [10] [13] , Cu3Si as a current collector, and amorphous SiO surrounding each Si grain to prevent cracking of Si.
We postulate that a composite material consisting of a poly-Si, Si oxide, Si-Cu alloy, Si-Li alloy, and Li-silicate oxide in a composite particle will likely
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Figure 6. Structural image of the Si, Li2O, and CuO composite as an aggregate of Si and Cu3Si nanoscale grains on amorphous silicon monoxide with a Li-silicate oxide compound (likely Li2SiO3) and a Li-Si alloy (likely Li4Si).
yield a good performance with respect to the charge-discharge characteristics in an LIB. The alloying of Li or Cu by mechanochemical processing can transform Si into a Si alloy or other oxide compound. Improvements in the stability of the Si anodes have been observed from such individual attempts, leading to improvements in the LIB performance [38] [39] [40] . We consider that further significant improvements will be obtained from synergistic effects when two or more approaches are simultaneously introduced in one simple operation.
4. Summary and Outlook
We produced a model for the high electrochemical performance of LIBs employing a Si, Li2O, and CuO composite as an anode material. A high coulombic efficiency of over 100% and long cycling stability at a high loading current density of 3000 mAh/g were achieved, indicating that the composite acted as an active material with dispersed Si nanoscale grains or Li-Si compounds. Li-Si or Cu-Si alloys are expected to function electrochemically in the charge-discharge process. The Cu3Si alloy in the prepared composite helps to improve the electrical conductivity; Li-Si compounds, which include Li4Si and Li2SiO3, work to improve the electrical conductivity and compensate for the lack of Li ions in the composite during charge-discharge cycling. The diffusion of Li+ into the composite that includes nano-Si grains, SiO, Li2SiO3, Cu3Si, and Li4Si in (a) of Figure 7 results in an initial charge of the LIB, and an alloy of Li-Si or SiO is synthesized in the composite in (b), as well as an irreversible Si-Li alloy, which does not participate in the charge-discharge process. Furthermore, the Li ions remove SiO and some Li4Si from nano-Si in (c), and the composited Li4Si is transformed to Li4-ySi. In (d), Li4-ySi receives Li atoms from Li2SiO3, which has a higher free energy of formation. The next charge occurs in (e). Li2SiO3 and Li4Si in the composite may not be the actual compounds found in the composite because
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Figure 7. Model of a charge-discharge mechanism employing the composite with Si, Li2O, and CuO to satisfy the high capacity and long cycling stability of the active anode material.
each material is assumed to be mixed as amorphous materials. Nonetheless, compounds in the composite that include Li atoms, such as Li2SiO3 and Li4Si, are consumed before and after the charge-discharge process, and Li atoms are expected to be available to compensate for the lack of Li ions in the anode of the LIB.
We conclude that the structure of the composite unites the structural features of an active material based on a Si composite with a high capacity and cyclic reversible charge properties. The following observations were made:
1) A homogeneous dispersion of Cu3Si nanoscale grains, Si nanoparticles, amorphous SiO, and Si-Li compounds was obtained using a simple mechanochemical process. The crystals of both Cu3Si and Si nanoparticles align randomly when ground at 27˚C in air.
2) Our prepared composite delivered a better capacity retention and coulombic efficiency (over 100%) than Si alone. The ground samples exhibited a better cycling ability than the simple mixtures, and we could show that the existence of a Si-Cu alloy, SiO, and Si-Li compounds in the composite are important factors for improving the cycling performance. Moreover, the electrochemical stability of Si with Cu3Si, Li2SiO3, and Li4Si synthesized by a ball milling process was important in controlling the cycling performance. We expect the controlled release of Li from the Li compounds formed in the composite to compensate for the lack of Li ions and Si oxide compounds in the composite and thus optimize the cycling performance.
Thus, we synthesized an active composite material to serve as an anode in an LIB using a simple mechanochemical process. Successful material preparation for a Si anode was performed with a smaller addition of CuO as a nonstoichiometric case, expanding our understanding toward mechanochemical processes. While optimizing our treating conditions for preparing the Si composite, we considered the different applications of nonstoichiometric reactions to other similar issues.
Acknowledgements
We kindly thank DOWA Holdings Corporation for their help in the construction and electrochemical measurements of the composite employed in this study.