Application of Graphene Prepared by Ultrasonic Exfoliation Method as a Conductive Additive in SiO Anodes for Lithium-Ion Batteries

Abstract

In this study, graphene produced via the ultrasonic exfoliation method was applied as a conductive aid for a SiO anode. The SiO anode with multilayer graphene demonstrated superior cycling, high-rate charge, and high-rate discharge properties, despite the resistivity of the active material layer being slightly higher than that of conventional AB. These properties are due not only to the electronic conductivity of graphene but also to effects related to its shape. A comparison of two types of graphene with different size produced via the same manufacturing method confirmed that graphene with a smaller size is superior in terms of the number of cycles, high rate-of-charge, and high rate-of-discharge; this may be due to the difference in the number of conductive paths formed within the active material layer due to the difference in the number of graphene particles per weight.

1. Introduction

The general-purpose rechargeable batteries available on the market include lead-acid, nickel-metal hydride (Ni-MH), nickel-cadmium (Ni-Cd), and lithium-ion batteries. Among these batteries, lithium-ion batteries are experiencing rapid growth in global demand owing to their compact size, light weight, high voltage, and lack of memory effects. These batteries are extensively used as crucial power sources in various electronic devices, including cell phones,¹ electric vehicles (EVs),² and stationary power supplies.³

Graphite is primarily used as the anode material in commercial lithium-ion batteries. The theoretical capacity of graphite is estimated to be 372 mAh g⁻¹,^4,5 and it is broadly classified into natural graphite and artificial graphite. Both types are characterized by high initial Coulombic efficiency, excellent cycle performance (charge–discharge cycle stability), and minimal volume change during charging and discharging.

As the applications of lithium-ion batteries expand, the demand for higher capacity and output increases. However, as the technological optimization of graphite-based anodes continues to advance, their actual capacities approach the theoretical values. Therefore, the potential for significant enhancements in the energy density of batteries using graphite-based anodes is becoming increasingly limited.

Therefore, silicon-based materials such as Si,^6–8 SiO,^9,10 Si-based material/carbon composites,^11,12 and silicon alloys¹³ are attracting attention as next-generation anode active materials. For example, Si is distinguished by its exceptionally high theoretical capacity (4200 mAh g⁻¹ as Li_4.4Si⁶ and 3579 mAh g⁻¹ as Li_3.75Si⁷) and low operating potential (≈0.4 V vs. Li/Li⁺).⁶ Moreover, silicon is the second most abundant element in the Earth’s crust and offers additional advantages, such as environmental friendliness. For these reasons, silicon-based materials have long attracted attention as anode active materials for lithium-ion batteries.

However, the significant volume change that occurs during charging and discharging presents a major challenge for cycle performance.¹⁴ Another significant issue with silicon-based anodes is their low electronic conductivity. The conductivity of silicon is substantially lower than that of graphite, which is extensively used in the anodes of lithium-ion batteries. The conductivity aids used in lithium-ion battery electrodes are intended to reduce the electrical resistance of the electrodes, and carbon black and graphite are mainly used. However, when active materials with large volume changes during charging and discharging, such as silicon, are used, the conductive network is easily destroyed and difficult to maintain (Fig. 1a). For the issue of resistance reduction of SiO, Tin (Sn) is added to the SiO₂ matrix in SiO to increase the conductivity of SiO itself.^15,16

Figure 1.

Effects of using graphene as a conductive material: (a) conventional conductive material (conventional methods) and (b) graphene as a conductive material (proposed method).

On the other hand, nanocarbon materials with unique shapes, such as carbon nanotubes (CNTs),¹⁷ vapor-grown carbon fibers (VGCFs),¹⁸ and graphene,¹⁹ are being considered as conductive auxiliaries for silicon-based anodes. Owing to their unique shapes and high conductivity, these materials are expected to maintain a conductive network during charge–discharge cycles and consequently contribute to a longer life of the silicon-based anode.

Graphene-based silicon-based anodes are expected to offer several advantages because of their unique properties. Specifically, these anodes may exhibit superior performance in the following two aspects. 1) Maintenance of the conductive network by graphene interposition: The structure of graphene interposed between silicon particles takes advantage of the high mechanical flexibility of graphene. Even when the volume of the material changes due to charging and discharging, the flexibility of graphene prevents the conductive network from being disconnected, facilitating the maintenance of the conductive network (Fig. 1b). This can reduce the decline in battery capacity even after repeated charging and discharging, resulting in a longer battery life. 2) Reduced resistance due to surface contact: The electrode obtained by mixing graphene and silicon-based powder has a structure in which graphene is in surface contact with the surface of the silicon-based particles. This surface contact is expected to have the advantage of lower resistance than conventional conductive aids with point contact. This will reduce the internal resistance of the battery. This property is especially valuable in applications that require rapid charging and high-power discharge.

Known methods for synthesizing graphene include mechanical exfoliation from graphite,²⁰ reduction of graphene oxide powder,²¹ pyrolysis of SiC,²² and chemical vapor deposition (CVD) using metal catalysts.²³ However, these methods either lack scalability for mass production or often synthesize not only multilayer graphene but also thin-film graphite. In addition, Graphene obtained by graphene oxide reduction or CVD method may have many defects.²⁴

The ultrasonic exfoliation^25,26 method produces graphene by irradiating a dispersion containing graphite powder with ultrasonic waves. When the dispersion is irradiated with ultrasound, bubbles are generated, and exfoliation is believed to be caused by the cavitation force generated when the bubbles collapse.²⁷ This method is relatively easy and inexpensive for producing graphene with low defects and is expected to have potential for scale-up. And there is no report on the application of graphene prepared by ultrasonic exfoliation to SiO anodes.

In this study, we verified the application of graphene produced by the ultrasonic peeling method as a conductive additive for SiO anodes. Specifically, two types of graphene with different size obtained by ultrasonic crushing of graphite were used as conductive auxiliaries. In addition, we report the results of an experiment using acetylene black (AB) for comparison.

2. Methods

2.1 Analysis of multilayer graphene

The two types of multilayer graphene used in this experiment were small sized graphene (XP) and large sized graphene (GP), supplied by 2D Materials Pte. Ltd. (2DM). These graphene were produced by ultrasonication of a liquid containing graphite particles followed by extraction of the supernatant liquid; AB is a carbon black produced by DENKA via pyrolysis of acetylene gas. A Keyence VHX-D500 scanning electron microscope (SEM) was used to observe each powder. Raman spectroscopy measurements were performed via a JASCO RMP-300 instrument with an excitation wavelength of 532.05 nm and two integrations. X-ray diffraction (XRD) measurements were performed via a Rigaku Ultima-IV instrument with CuKα (1.54184 Å) as the X-ray source at 40 kV and 40 mA with a scan speed of 5° min⁻¹. Graphene in powder form was measured in air.

Graphene in powder form is difficult to handle because it tends to scatter in air. On the other hand, a large amount of dispersion is required to obtain the required amount of graphene at low concentrations. For these reasons, it would be desirable for the graphene used in electrode manufacturing to be highly concentrated. Therefore, to confirm the dispersibility of graphene, the particle size distribution of the graphene dispersion was measured via the ultrasonic attenuation method.²⁸ For these measurements, N-methyl-2-pyrrolidone (NMP) was used as a medium for dispersion, and graphene dispersions were adjusted to 1 wt% and 5 wt% concentrations as samples.

2.2 Resistivity evaluation

To investigate the effects of XP, GP, and AB carbon powders on the conductivity of the active material layer, SiO (Osaka Titanium, particle size of 4 µm), a PI binder (UBE, U-Varnish A), and each conductivity aid were blended at a solid composition ratio of 80/5/15 wt% and mixed with NMP in a self-rotating mixer (THINKY, ARE-310 Awatori-Rentaro). A slurry with a solid ratio of 62 wt% was produced in this process. The mixing conditions were 2000 rpm for 40 minutes. To accurately measure the resistivity of the active material layer, the slurry was applied with an applicator (NOGAMIGIKEN) on a glass substrate (MATSUNAMI-GLASS, 1.1 mm thick), avoiding highly conductive current collectors, dried with hot air at 80 °C, and then vacuum heat treated at 300 °C for 3 h. This treatment was performed in a glass tube oven (BUCHI, B-585TO). Electrical resistivity was measured in a dry booth (NIHON-SPINDLE, NS-Dry booth) at a dew point of approximately −70 °C and a room temperature of 20 °C using a MITSUBISHI CHEMICAL Loresta-EP MCP-T360 with a 4-terminal, 4-probe method.

To investigate the effect of each conductivity aid on the volume resistivity of the powder, three levels of mixed powder (94/6 wt%) consisting of XP, GP, and AB carbon powders and SiO powder were prepared by mixing at 100 rpm for 1 h via an automatic mortar (NITTO-KAGAKU, ANM-140D). At room temperature (20 ± 2 °C), the mixed powder was pressurized in the range of 1–20 kN, and the pressed density and volume resistivity were measured. A volume resistivity measuring device (NITTOSEIKO ANALYTECH, MCP-PD600) was used to measure the volume resistivity of the mixed powder.

2.3 Fabrication of test electrodes and charge–discharge tests

For the SiO anode, a slurry was prepared under the same conditions used for the resistivity measurement, and this slurry was applied to a nickel-plated steel foil (NIPPON STEEL, SP-Nickel, 10 µm thick). The test electrode was then dried with hot air at 80 °C and vacuum heat treated in a glass tube oven at 300 °C for 3 h. The weight per unit area of the active material layer of the SiO anode was 0.25 ± 0.01 g m⁻². For comparison, a SiO anode with AB as a conductivity aid was also prepared in the same way.

The reason why the PI binder was used for the SiO anode was to suppress the exfoliation of the active material layer from the current collector, even with large volume changes in the active material due to charging and discharging, and to facilitate reversible capacity. The binder has excellent heat resistance, mechanical strength, and chemical stability. During electrode production, polyamic acid (polyimide precursor) was dissolved in NMP as a binder and applied to the current collector, followed by dehydration and an imidization reaction. This process improves the mechanical strength and bonding of the silicon-based anode, suppresses active material exfoliation and pulverization, and achieves good cycling performance. However, the dehydration and imidization processes require high-temperature heat treatment at 200–400 °C. In addition, the active material and current collector were heated under vacuum to prevent oxidation.

To assemble the test cell, a SiO anode (φ11 mm) was used as the test electrode. Lithium metal foil (Honjyometal, 500 µm thick, φ14 mm) was employed as the counter electrode, and a glass filter (ADVANTEC, GA-100, φ16 mm) and a polyolefin microporous membrane (Celgard, Celgard2325, φ16 mm) were overlaid as the separator. LiPF₆/(ethylene carbonate : diethyl carbonate = 1 : 1 vol + 1 wt% vinylene carbonate) (1 mol L⁻¹) (Kishida chemical, battery grade, 0.15 ml) was used as the electrolyte, and the coin cell was assembled in a dry environment (NIHON-SPINDLE, NS-Dry booth) with a dew point of approximately −70 °C. Since the main purpose of this experiment was to evaluate the performance of the anode, the influence of the cathode was eliminated as much as possible by using a lithium metal foil as the counter electrode.

In the cycle test, a charge/discharge device (ELECTROFILD, EF-7200) was used to perform 100 cycles of constant current (CC)-charging and CC-discharging at a cutoff voltage of 0.001 V to 1.5 V and a charge/discharge current rate of 0.2 C-rate (6 A m⁻²) in a 30 °C environment. A rest period of 10 minutes was provided between charging and discharging. In the high-rate charge test, the silicon-based anode was activated by repeating 3 cycles of charge–discharge under the conditions of a cutoff voltage of 0.001 V to 1.5 V and a CC-charge/CC-discharge current of 0.2 C-rate in a 30 °C environment. The batteries were then charged at a current rate of 0.2 C-rate at 0.001 V and further CC-charged at current rates of 0.2 C-rate, 0.5 C-rate (15 A m⁻²), 1.0 C-rate (30 A m⁻²), and 2.0 C-rate (60 A m⁻²) until 1.5 V was reached. In the high-rate discharge test, the silicon-based anode was activated by repeating 2 cycles of charging and discharging at a cutoff voltage of 0.001 V to 1.5 V and a CC-charge/CC-discharge current rate of 0.2 C-rate at 30 °C. After that, discharge was performed at a current rate of 0.2 C-rate to 1.5 V and then at current rates of 0.2 C-rate, 1.0 C-rate, 2.0 C-rate, and 3.0 C-rate (90 A m⁻²). CC-discharges were conducted at current rates of 0.2 C-rate, 1.2 C-rate, 2.0 C-rate, and 3.0 C-rate until 0.001 V was reached.

2.4 Fabrication of test electrodes and charge–discharge tests

After the cycle test, the coin cell was disassembled via a hydraulic coin cell disassembler (MTI, MSK-110D) and cleaned with dimethyl carbonate (DMC), and the cross section of the anode was observed via scanning electron microscopy (SEM). Since the anode after charging and discharging is prone to react with moisture in the atmosphere and may be altered over time, the disassembly of the coin cell was performed in a dry environment, as in the coin cell fabrication process. For observation of the cross section via SEM, the test electrode was punched out via a hand punch (Nogamigiken, quadrangular type). After cross-sectioning, the sample was subjected to SEM in a dry environment for observation.

3. Results

3.1 Analysis of multilayer graphene

Figures 2a to 2c show SEM images of XP, GP, and AB powders used as conductive materials (voltage of 10 kV, magnification of 10000), respectively. The lateral size of XP ranged from 1–3 µm (Fig. 2a), and that of GP ranged from 1–4 µm (Fig. 2b). According to the catalog of the conductivity aid manufacturer, XP has an average specific area of 100–140 m² g⁻¹ and an average lateral size of 1–2 µm, whereas GP has an average specific area of 90 m² g⁻¹ and an average lateral size of 1–3 µm. For AB, the average particle size is 35 nm (primary particle form), and the average specific area is 68 m² g⁻¹. Figures 2d to 2g show the XRD patterns of XP, GP, and AB. In the XRD patterns of XP (Fig. 2d) and GP (Fig. 2e), a peak originating from the (002) plane of the graphite crystal was observed at approximately 26.5 degrees. Specifically, the peak of (002) in XP was located at 26.52 degrees, and that in GP was located at 26.62 degrees; the peak in XP was shifted to the lower angle side compared to that in GP (Fig. 2g), indicating that the interlayer distance in XP is wider than that in GP. This result suggests that XP is a graphene with thinner layers than GP. Figure 2h shows the Raman spectra of XP, GP, and AB. In these samples, the G-band (approximately 1580 cm⁻¹) and 2D-band (approximately 2700 cm⁻¹) were observed; the G-band is a common feature observed not only in graphene but also in graphite, CNTs, and other carbon materials with sp² bonds. In graphene, it has been reported that the peak of the G-band tends to shift toward lower frequencies as the number of layers increases.²⁹ The D-band (approximately 1350 cm⁻¹), which originates from defects in graphene (such as graphene edges and in-plane lattice defects), was observed at a lower intensity in XP and GP than in AB. The intensity ratio of the 2D-band to the G-band (I_2D/I_G) correlates with the number of graphene layers.³⁰ This ratio was measured to be 0.32 for XP, 0.28 for GP, and 0.40 for AB. Furthermore, the intensity ratio of the D-band to the G-band (I_D/I_G) was 0.43 for XP, 0.51 for GP, and 0.85 for AB. These results suggest that XP has fewer layers and defects compared to GP.

Figure 2.

SEM images ((a) to (c)), XRD patterns ((d) to (g)), and Raman spectra ((h)) of verified conductive aids. (a) & (d) XP, (b) & (f) GP, (c) & (f) AB, and (g) comparison of the XRD spectra of the three samples.

Figure 3 shows the results of particle size measurements for XP and GP using the ultrasonic attenuation method. Comparing Figs. 3a and 3b, it is evident that the particle size of XP is smaller than that of GP. Additionally, as seen in Fig. 3a, the sample with smaller graphene (XP) does not show any change in particle size even when the graphene concentration in NMP is increased. On the other hand, as shown in Fig. 3b, the sample with larger graphene (GP) tends to have larger particle sizes as the concentration in NMP increases. This suggests that GP, due to its larger particle size, is more prone to aggregation.

Figure 3.

Measurement results of graphene via ultrasonic attenuation spectroscopy: (a) results of thinner graphene (XP) and (b) results of thicker graphene (GP).

3.2 Resistivity evaluation

Table 1 shows the electrical resistivity of silicon-based active material layers with XP, GP, and AB as conductive aids, as measured by the 4-terminal, 4-probe method. XP had a lower resistivity than GP; this may be due to the difference in the number of particles per weight of graphene, resulting in a difference in the number of conductive paths formed between the graphene particles in the active material layer.

Table 1. Electrical resistivity of SiO anode active material layer. SiO/AB/PI ratio = 80/5/15 wt%. 4 probe method using Loresta-EP MCP-T360.

	Electrical resistivity
XP	0.218 × 10−1 Ω cm
GP	0.258 × 10−1 Ω cm
AB	0.199 × 10−1 Ω cm

Figure 4 shows the relationship between the pressed density and volume resistivity of a mixed powder consisting of carbon and SiO powders (94/6 wt%). For all the powder mixtures, there was a tendency for the density to increase and the volume resistivity to decrease with increasing compression by mechanical pressing. However, the degree of this decrease varies from powder to powder. When the density of the powder was less than 1.2 g cm⁻³, AB had the lowest volume resistivity, and with increasing density, the resistivity of graphene became lower than that of AB. For XP and GP, with increasing density above 1.25 g cm⁻³, XP had a lower volume resistivity than AB. However, when comparing XP and GP at a density of 1.25 g cm⁻³, XP had a lower resistivity; this may be due to the difference in the number of particles per weight of graphene, which resulted in a difference in the number of conductive paths formed between the graphene particles in the active material layer.

Figure 4.

Relationships between the pressed density and volume resistivity of a mixed powder consisting of carbon and SiO powders (94/6 wt%). (a) XP, (b) GP, and (c) AB.

3.3 Fabrication of test electrodes and charge–discharge tests

Figure 5 shows the results of charge–discharge tests conducted repeatedly at a current rate of 0.2 C-rate. In this test, “discharge” means that the active material releases lithium ions, i.e., undergoes a dealloying reaction. Conversely, “charging” means that the active material absorbs lithium ions, i.e., undergoes a Li-alloying reaction. The Coulombic efficiencies calculated based on the initial charge and discharge capacities were 68.49 % for (a), 67.05 % for (b), and 69.88 % for (c). As shown in Fig. 5c, the silicon-based anode with AB as a conducting aid had a reversible capacity of half of the initial capacity after 100 cycles. However, as shown in Figs. 5a and 5b, the silicon-based anode with graphene replacing the conductive aid did not show the significant capacity drop observed for the anode in (c). In Fig. 5d, the charge and discharge capacities for each cycle are plotted. The anode with graphene, i.e., (a) and (b), maintains a more stable reversible capacity than the anode with only AB, i.e., (c). These results indicate that the replacement of AB with graphene allows silicon-based electrodes to maintain the conductive network of the electrode even when large volume changes in the active material occur during charging and discharging. Even with graphene made via the same manufacturing method, when comparing (a) and (b), the anode of (a) with smaller graphene (XP) showed more stable cycle characteristics than the anode of (b) with larger graphene (GP); this clearly shows that the size of graphene affects the cycle characteristics of the silicon-based anode, suggesting that the electrode may exhibit more stable cycling performance with a smaller sized graphene.

Figure 5.

Relationships between capacity and voltage: (a) XP, (b) GP, and (c) AB. (d) Charge–discharge characteristics.

Figure 6 shows the results of high-rate charging tests when charging was performed at current rates of 0.2 C-rate, 0.5 C-rate, 1.0 C-rate, and 2.0 C-rate. For the anode using XP, the charge capacity at 0.2 C-rate was 1419.6 mAh g⁻¹, that at 0.5 C-rate was 1214.5 mAh g⁻¹, that at 1.0 C-rate was 1019.0 mAh g⁻¹, and that at 2.0 C-rate was 583.6 mAh g⁻¹ (Fig. 6a). For the anode using GP, the charge capacity at 0.2 C-rate was 1377.6 mAh g⁻¹, that at 0.5 C-rate was 1156.0 mAh g⁻¹, that at 1.0 C-rate was 937.4 mAh g⁻¹, and that at 2.0 C-rate was 350.4 mAh g⁻¹ (Fig. 6b). For the anode with AB, the charge capacity at 0.2 C-rate was 1431.3 mAh g⁻¹, that at 0.5 C-rate was 1208.4 mAh g⁻¹, that at 1.0 C-rate was 974.6 mAh g⁻¹, and that at 2.0 C-rate was 390.2 mAh g⁻¹ (Fig. 6c). These charging capacities are summarized in Fig. 6d.

Figure 6.

Relationships between capacity and voltage when the charge rate (C-rate) is changed: (a) XP, (b) GP, and (c) AB. (d) Relationships between C-rate and charge capacity.

Figure 7 shows the results of high-rate discharging tests when discharging was performed at current rates of 0.2 C-rate, 1.0 C-rate, 2.0 C-rate, and 3.0 C-rate. For the anode using XP, the discharge capacity at 0.2 C-rate was 1384.0 mAh g⁻¹, that at 1.0 C-rate was 1336.7 mAh g⁻¹, that at 2.0 C-rate was 1251.0 mAh g⁻¹, and that at 3.0 C was 1147.1 mAh g⁻¹ (Fig. 7a). For the anode using GP, the charge capacity at 0.2 C-rate was 1333.7 mAh g⁻¹, that at 1.0 C-rate was 1299.5 mAh g⁻¹, that at 2.0 C-rate was 1182.2 mAh g⁻¹, and that at 3.0 C-rate was 950.5 mAh g⁻¹ (Fig. 7b). For the anode using AB, the charge capacity at 0.2 C-rate was 1301.9 mAh g⁻¹, that at 1.0 C-rate was 1238.3 mAh g⁻¹, that at 2.0 C-rate was 1072.8 mAh g⁻¹, and that at 3.0 C-rate was 906.1 mAh g⁻¹ (Fig. 7c). These charging capacities are summarized in Fig. 7d. Figures 6d and 7d show that XP has higher capacities than GP and AB, even when charging or discharging at higher rates.

Figure 7.

Relationships between capacity and voltage when the discharge rate (C-rate) is changed: (a) XP, (b) GP, and (c) AB. (d) Relationships between C-rate and discharge capacity.

According to Yamada et al., the following reaction equation is proposed as a reaction for charging and discharging SiO.³¹

  

\begin{equation} \text{4SiO} + \text{17.2Li}\to \text{3(Li$_{4.4}$Si)} + \text{Li$_{4}$SiO$_{4}$}\Leftrightarrow \text{3Si} + \text{13.2Li} + \text{Li$_{4}$SiO$_{4}$} \end{equation} (1)

During initial charging (Li storage process), Li_4.4Si and Li₄SiO₄ are generated, and this Li₄SiO₄ is said to be a factor in the initial irreversible capacity. When discharging (Li release process) is performed after the initial charge, Li_4.4Si becomes Si, and the reaction Li_4.4Si $ \Leftrightarrow $ Si occurs repeatedly in the Li₄SiO₄ matrix thereafter. It has also been suggested that SiO reacts with Li during the charge-discharge process to produce Li₂Si₂O₅, Li₂SiO₃, Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄, and Li₁₅Si₄ in addition to Li_4.4Si and Li₄SiO₄.^32–34

Furthermore, Sachi et al., calculated the relationship between Li absorption and the rate of volume change in SiO considering the volume density of each product, suggesting that in the initial stage of Li absorption, the volume change is slight, but in the reversible capacity region, the volume changes almost linearly in proportion to Li absorption until the volume increases by 2.5 times.³⁵

Figure 8 shows the appearance of the electrode after 100 cycles at a current rate of 0.2 C-rate. For the anode with graphene, the active material layer did not peel off from the current collector even after washing with DMC (Figs. 8a and 8b). On the other hand, for the anode using AB, the active material layer peeled off from the current collector during disassembly (Fig. 8c). It has been reported that the use of graphene as a filler improves various physical properties, including mechanical strength,^36,37 and in the SiO anode, the mechanical strength of the active material layer is also improved; as a result, delamination is suppressed. This is understood to be due to an effect of the shape of graphene rather than its electronic conductivity. Figures 8d to 8g show SEM images of the cross section of the electrode after 100 cycles at 0.2 C-rate compared with that before charging and discharging. The electrode with AB could not be observed via SEM because it was detached during cell disassembly. The thickness of the active material layer after 100 cycles increased by approximately 173 % for XP and 178 % for GP, compared to the thickness of the active material layer before charging/discharging. However, the volume resistivity of a conductive paste containing silver (Ag) powder in flake form applied on a substrate of stretchable chloropropylene rubber and dried samples stretched to 100 to 150 % was measured via the four-terminal method.³⁸ As the deformation ratio increases, the resistivity also increases; however, the use of flake-shaped, thin Ag powder has been shown to suppress the increase in resistance. In the literature, flake-shaped Ag powder has been used, but graphene is also flake shaped. Therefore, the increase in resistance due to the volume expansion of the active material layer during charging may be suppressed.

Figure 8.

(a)–(c) Photos of the anode after charging and discharging: (a) XP, (b) GP, and (c) AB. (d)–(e) SEM images of the cross section using the conductive aid of XP: (d) before charging and discharging and (e) after charging and discharging. (f)–(g) SEM images of the cross section using the conductive aid of GP: (f) before charging and discharging and (g) after charging and discharging.

4. Conclusion

In this study, graphene produced through ultrasonic exfoliation method was applied as a conductive aid for a SiO anode. The SiO anode with the graphene demonstrated superior cycling, high-rate charge, and high-rate discharge properties, although the resistivity of the active material layer was slightly greater than that with conventional AB. These properties are understood to be due not only to the electronic conductivity of graphene but also to effects based on its shape. It was confirmed that graphene with fewer layers (XP) was superior in terms of cycling, high rate-of-charge, and high rate-of-discharge; this may be due to the difference in the number of conductive paths formed within the active material layer due to the difference in the number of graphene particles per weight. In silicon-based anodes, the volume change of the active material during charging and discharging is large, and as a result, the electronic network of the electrode is easily disconnected. To address this problem and achieve reversibly stable electrode properties, it is important to effectively form a network between conductive particles. For this purpose, we believe that the use of conductive particles with a flake shape that allows surface connection is preferable. Owing to the excellent properties of graphene, such as electrical conductivity, mechanical strength, and chemical stability, silicon-based anodes with high capacity, long life, and high input/output characteristics can be realized.

CRediT Authorship Contribution Statement

Yoshiharu Ajiki: Conceptualization (Lead), Data curation (Lead), Formal analysis (Lead), Project administration (Lead), Writing – original draft (Lead)

Taichi Sakamoto: Data curation (Supporting), Writing – review & editing (Supporting)

Yuta N. Ikeuchi: Data curation (Supporting), Writing – review & editing (Supporting)

Naoto Yamashita: Data curation (Supporting), Writing – review & editing (Supporting)

Takashi Mukai: Conceptualization (Equal), Data curation (Supporting), Writing – review & editing (Supporting)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Footnotes

The content of this paper has been published as a preprint in Research Square. URL: https://doi.org/10.21203/rs.3.rs-4215611/v1

T. Mukai: ECSJ Active Member

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