Mechanochemically Metamorphosed Composites of Homogeneous Nanoscale Silicon and Silicate Oxides with Lithium and Metal Compounds

An active anode material for Li-ion batteries was synthesized using a simple mechanochemical process to minimize the large change in Si volume observed during charge-discharge operation and to compensate for the associated irreversible loss of Li or irreversible capacity loss, which are obstacles to achieve high-performance electrochemical properties during charge-discharge. The composite was mechanochemically milled with Si, lithium oxide, and copper oxide as raw materials; the composite contains Si nanoparticles, amorphous silicon monoxide, and Si-Li or Si-Cu alloy compounds, and it exhibits improved electrochemical properties. In particular, this composite achieved a better capacity retention, higher coulombic efficiency (over 100%), and longer cycling performance than Si alone, indicating considerable optimization of the electrical and ionic conductivity in the composite. The developed method allowed for control of the Li content to compensate for the lack of Li ions in the composite, and the cycling performance was optimized using the Cu alloy, oxide, and Li compounds within the composite.


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 [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, Li 2 O, 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, Li 2 O, 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.

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/cm 2 .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.

Assembly of a Test Coin Cell for the LIB
The coin cells were assembled in an Ar-filled glove box using   .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.

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 SiO 2 , 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.
Furthermore, the mechanochemical reaction between Si and Li 2 O 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.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 Cu 3 Si [33] and acts as a current collector; this alloy is electrically conductive and an important material    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.

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, Li 2 O, 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 irre- The specific gravity of the prepared composite is approximately 2.12 g/cm 3 , which is less than the specific gravity of Si (2.33 g/cm 3 ).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 Li 4 Si) 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, Li 2 O, and CuO composite is confirmed as an aggregate of nanoscale grains based on Si, Cu 3 Si, 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], Cu 3 Si 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 provements 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.

Summary and Outlook
We produced a model for the high electrochemical performance of LIBs em- 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.

N.
Shimoi et al.DOI: 10.4236/msa.2018.91008112 Materials Sciences and Applications 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]

Figure 1 .
Figure 1.(a) Initial charge-discharge properties of the prepared composite with Si, Li 2 O, and CuO at loading current densities of 0.01 mA/cm 2 (0.003 C), 3.2 mA/cm 2 (1 C), and 9.6 mA/cm 2 (3 C); (b) Comparison of the cycling properties of the anode with the prepared Si, Li 2 O, and CuO composite (red line); Si and CuO composite (blue line); Si and Li 2 O composite (green line); Si, Li 2 O, and CuO mixture (dotted line); and Si particles as a reference (black line).The attenuation observed for the Si, Li 2 O, and CuO composite after 800 cycles is due to peeling of the film from the Cu foil.

Figure 1 (
Figure 1(a) shows the electrochemical performance of the composite anode evaluated in a 2032-type coin cell with a LiCoO 2 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/cm 2 (0.003 C), 3.2 mA/cm 2 (1 C), and 9.6 mA/cm 2 (3 C) at 27˚C.The state of the charge of the battery was set to 100%.An LIB anode composed of Si, Li 2 O, 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 indicatethat 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/cm 2 (0.003 C), as shown in Figure1(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/cm 2 (1 C) and 9.6 mA/cm 2 (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 Li 2 O 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 ag-

Figure 1 (
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/cm 2 (3 C) with LiCoO 2 employed as the cathode in a coin cell.The measured anodes employ a composite of Si, Li 2 O, and CuO produced by a mechanochemical process (red line), a composite of Si and CuO (blue line), a composite with Si and Li 2 O (yellow line), a mixture of Si, Li 2 O, and CuO (dotted yellow line), and Si particles as a reference (black line).The Si/Li 2 O/CuO and Si/CuO composites exhibited reversible capacities above 3000 mAh/g after 800 cycles; notably, the Si/Li 2 O/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/Li 2 O, which showed capacities below 50% after relatively few cycles.The Li 2 O 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 syn-

Figure 2
Figure 2 shows the crystallinity and ratio of components with Li atoms for the composite obtained by the mechanochemical process of Si, Li 2 O, and CuO.The oxide atoms in CuO are reduced, and residual Cu is expected to form an alloy

Figure 2 .
Figure 2. (a) XRD patterns of the Si, Li 2 O, 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 Li 2 SiO 3 or Li 4 Si (blue line), with minor LiOH (aqua line) and Li 2 O (black line) components.
for charge-discharge characteristics, which is determined by comparing the Si/Li 2 O/CuO composite with the Si/Li 2 O 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 Li 2 O. Thus, the compounds synthesized from the reduction and oxidation of CuO or Li 2 O 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 et al.DOI: 10.4236/msa.2018.91008117 Materials Sciences and Applications expected to exist in the Si, Li 2 O, 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 Li 2 SiO 3 , Li 4 Si, LiOH, and Li 2 O.The content ratio of the different Li components in the composite was calculated by deconvolution of the peak position, which was calibrated using C1s signal.Li was found mainly in the form of Li 2 SiO 3 or Li 4 Si 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 Li 2 O do not react with each other to form other components.

Li 2
SiO 3 has a higher free energy of formation than Li 2 O and easily decomposes to a Si-Li oxide or Si-Li alloy; furthermore, Li 2 SiO 3 does not function as an LIB active material.

Figure 3 Figure 3 .
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., Materials Sciences and Applications Japan).The SEM overview image (Figure 3(a)) reveals the composite particles obtained by the ball milling process with Si, Li 2 O, 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 Cu 3 Si in the composite [21].The crystal lattices indicate the orientation of each nanoscale Si or Li 4 Si, and Cu 3 Si 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, Cu 3 Si 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 Cu 3 Si grains.

Figure 4 .
Figure 4. Comparison of the impedance charts of the composites with Si, Li 2 O, and CuO (red line), Si and Li 2 O (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.

b
Materials Sciences and Applications 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.

versibleFigure 5 .
Figure 5. Distributions of Si and Cu by EDX and Si and Li by EELS in the composite with Si, Li 2 O, and CuO.((a) and (b)) SEM cross-sectional views of the composite with Si, Li 2 O, 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, Li 2 O, and CuO; EDX and EELS maps after 800 charge-discharge cycles at 3000 mAh/g.

Figure 6 .
Figure 6.Structural image of the Si, Li 2 O, and CuO composite as an aggregate of Si and Cu 3 Si nanoscale grains on amorphous silicon monoxide with a Li-silicate oxide compound (likely Li 2 SiO 3 ) and a Li-Si alloy (likely Li 4 Si).

Figure 7 . 2 )
Figure 7. Model of a charge-discharge mechanism employing the composite with Si, Li 2 O, and CuO to satisfy the high capacity and long cycling stability of the active anode material.

2.1. Composite of Si, Li2O, and CuO Particles
and in air to produce aggregates composited with Si, Li 2 O, and CuO.We then established a new anode composite to exploit the anticipated mechanochemical reaction between Si, Li 2 O, and CuO when ground together.We designed particles N. Shimoi et al.DOI: 10.4236/msa.2018.91008113 Materials Sciences and Applications