Electrodeposited Si ˗ O ˗ C as a High-Rate Performance Anode for Li ˗ ion Capacitor

Li ˗ ion capacitors (LIC) which bridge the advantages of supercapacitors and Li ˗ ion batteries have attracted a great deal of attention as a promising energy storage device. In this study, we synthesize the Si ˗ O ˗ C by electrodeposition at low levels of electricity below 2.0 C cm − 2 as an anode for LIC. The deposited Si amounts are controlled by charge density during electrodeposition from 0.3 to 1.0 C cm − 2 . The material and electrochemical characteristics of Si ˗ O ˗ C fabricated at a charge density of 1.0 C cm − 2 are studied. The LIC consisting of Si ˗ O ˗ C anode shows an encouraging 95% retention of the initial capacity after 1000 cycles. In addition, the LIC shows a capacity retention ratio of 94% at 140 C ˗ rate. This study reveals the potential prospect to use Si ˗ O ˗ C fabricated by electrodeposition as an anode for a high ˗ rate capability for LIC.

Li˗ion batteries (LIB) and supercapacitors are extensively used as an electrochemical energy storage device for electric and industrial devices because of their advantages, such as high energy of LIB and power density of supercapacitors (LIB: 150˗200 W h kg −1 , supercapacitors: >1 kW kg −1 ). [1][2][3][4][5][6] As the electric vehicle industry develops, there is a growing need to develop alternative energy storage devices which have both the advantages of LIB and supercapacitors. To meet such requirements, Li˗ion capacitors (LIC) have been widely studied as a promising energy storage device to bridge the gap between the high˗energy of LIB and the high˗power of supercapacitors, since it exhibited a longer cyclability and higher power density than LIB as well as a higher energy density than supercapacitors. [7][8][9][10][11] Silicon is an attractive alternative due to its high theoretical capacity of 4200 mAh g −1 (fully lithiated phase, Li 4.4 Si). Furthermore, Si and Si˗based materials have many strong points as an anode such as lightweight, low cost benefits, and a possibility to lithiate at low˗potential (E Lithiation < 0.5 V vs. Li/Li + ). For these reasons, Si has been widely researched and applied as an anode for electrochemical energy storage devices. However, despite this advantage, Si suffers from volume change during lithiation/de˗lithiation during initial cycles, resulting in degradation of electrochemical performance. For that reason, other materials such as hard carbon, 12 graphite, [13][14][15][16] carbon nanofibers, 17 graphene, 18 and Li 4 Ti 5 O 12 19 have been widely used as an anode for LIC.
In previous reports, we have already studied a Si-based composite consisting of O and C, namely Si˗O˗C hereafter, by electrodeposition at a charge density of 2.0 C cm −2 in the organic solution. 20,21 The Si˗O˗C anode exhibited a delithiation capacity of 842 mAh g −1 for 7200 cycles with good discharge capacity retention ratio. These beneficial electrochemical properties are attributed to the uniformly dispersed SiO x in Si˗O˗C and the homogeneous distribution of Si, O, and C which can act as a buffer to decrease the internal stress during lithiation/de˗lithiation. Despite this remarkable cyclability, it was not easy to assemble full˗cell design for LIB because it is difficult to obtain sufficient amounts of deposited Si owing to the structural instability of Si˗O˗C. By contrast, in the LIC system, small amounts of Si˗O˗C could be used as an anode material because of the high theoretical capacity of Si and the outstanding cyclability of Si˗O˗C.
Herein, we investigated the possibility of Si˗O˗C as an anode for LIC full˗cell configuration with carbon nanotubes (CNT) cathode. The Si˗O˗C was deposited on a surface treated with Cu substrate with a lower charge density below 2.0 C cm −2 . The materials properties of Si˗O˗C fabricated at different charge densities of 0.3, 0.5 and 2.0 C cm ˗2 were measured by field˗emission scanning electron microscopy (FE˗SEM), laser scanning confocal microscope, X˗ray photoelectron spectroscopy (XPS), energy˗dispersive X˗ray spectroscopy (EDS), inductively coupled plasma (ICP), and by glow˗discharge optical emission spectroscopy (GDOES). For the full˗cell configuration, a carbon nanotube cathode was fabricated by electrophoretic deposition (EPD) using metal additive under high voltage conditions. 22,23 The LIC full˗cell consisting of Si˗O˗C anode and CNT cathode exhibited a good rate˗performance of 95% retention ratio for 1000 cycles and high rate˗capability of 94.3% capacity retention at 140 C˗rate. We believe that this study introduces the applicability of Si˗O˗C as a high˗rate performance anode for LIC.

Experimental
Synthesis of Si˗O˗C.-Before electrodeposition, the surface cleaning of as-received Cu substrate was conducted by ultrasonication using trichloroethylene and ethanol for 20 min, respectively. Afterwards, electrochemical degreasing was carried out. Next, the Cu substrate was soaked in an acid solution (10 vol% H 2 SO 4 ) for 10 sec and rinsed with distilled water, ethanol. The Si˗O˗C was synthesized by electrodeposition in an organic solution using Cu substrate, Pt coil, and Li piece as working, counter, and a reference electrode. Propylene carbonate (PC, Kishida) was used as the electrolyte for electrodeposition. Silicon tetrachloride (SiCl 4 , Sigma˗Aldrich) was employed as a Si source and tetrabutylammonium perchlorate purchased from Kanto˗Chemical was used as a supporting electrolyte. The electrodeposition was conducted using a beaker cell in a glove box in an argon atmosphere and a low dewpoint around −90°C.A˗1.0 mA cm −2 constant cathodic current was impressed to pass a charge of 0.3, 0.5, and 1.0 C cm −2 . More detailed conditions for electrodeposition of Si˗O˗C are described in previous reports. 20,21,23 Full˗cell configuration.-For the LIC full˗cell configuration, CNT cathode was fabricated by EPD at −120 V. Before the LIC test, capacity optimization was carried out by the half˗cell test. After capacity balancing, the LIC full˗cell was built in a glove box which is in an argon atmosphere (−110°C of dewpoint). The detailed information about the synthesis process of CNT film and capacity optimizing of LIC is provided in the supporting information.
Material and electrochemical characterization.-The morphological features of the Si˗O˗C was examined by FE˗SEM (Hitachi, S˗4800). The thickness of Si-O-C composite was measured by laser scanning confocal microscope (Keyence, VK˗X250). The elemental mapping was analyzed by SEM˗EDS (Hitachi TM˗3000). The Si amount was measured by ICP (Thermo Scientific, iCAP6500 Duo). The deposited CNT amounts were examined by ultra˗microbalance (Sartorius, SC2 ultra˗microbalance). For analyzing the surface chemical properties of Si˗O˗C composite, the sample was measured by XPS (JEOL, JPS˗9010TR) using a transfer vessel to prevent oxidization of Si˗O˗C composite. The elemental compositions of Si˗O˗C were measured by GDOES(Horiba, JY˗5000RF).
The Si˗O˗C and CNT half˗cells were tested using lithium metal as a counter electrode. Half˗ and full˗cells were built in a glove box which is in an argon atmosphere (dewpoint below −110°C). The half˗cell and LIC full˗cell were assembled in a liquid electrolyte consisting of 1.0 mol dm −3 of lithium˗perchlorate (LiClO 4 ) in propylene carbonate(Kishida) and ethylene carbonate (Kishida)(PC:EC = 50:50, v/v percent). Cyclic voltammetry (CV) of Si˗O˗C was tested at a scan rate of 0.1 mV s ˗1 with a voltage range of 0.01-1.20 V vs. Li/Li + (Biologic, VSP potentiostat). The charge/discharge characteristics of the assembled LIC full˗cell was performed at a current value of 100 mA g (of AC) −1 for 1000 cycles. The rate˗performance of the LIC full˗cell was tested with a different C˗rate condition from 0.1 to 180 C˗rate (1C is 20 mA g (of AC) −1 ). Figure 1a shows the chronopotentiogram of electrodeposition for the synthesis of Si˗O˗C with different charge densities from 0.3 to 1.0 C cm −2 . The current density of −1.0 mA cm −2 was applied to maintain the potential at around 1.2-1.3 V vs. Li/Li + during electrodeposition. In our previous report, it was revealed that the decomposition of SiCl 4 is carried out at 1.3 V vs. Li/Li + . 21 As seen in Fig. 1a, all samples demonstrated that the Si˗O˗C was deposited at 1.3 V vs. Li/Li + , which is in agreement with previous studies. 20,21 The amount of deposited Si was examined by ICP (Fig. 1b). The Si˗O˗C deposited at a charge density of 0.3, 0.5, and 1.0 C cm −2 has Si amounts of 9.0, 10.4, and 19.7 μg cm −2 , respectively. These values indicate that the amounts of deposited Si increased with increasing the quantities of electricity from 0.3 to 1.0 C cm −2 . To further investigate each electrodeposition stage, the surface characteristics of each sample were analyzed by XPS measurement. Figure 2 presents the XPS results of Si˗O˗C fabricated at a different charge density of 0.3, 0.5 and 1.0 C cm −2 with a spectra of Si 2p3/2 and Cu 2p3/2 . At 0.3 C cm −2 , the SiO x /Si peak was detected at 103.2 eV. As increasing the charge density for electrodeposition up to 1.0 C cm −2 , the new peak was slightly increased at 101.7 eV, which is attributed as the SiO peak. This finding demonstrates that the Si˗O˗C does not consist of metallic Si (binding energy of metallic Si: 99 eV). In the spectra of Cu 2p3/2 , the Si˗O˗C prepared at a charge density of 0.3 C cm ˗2 shows the Cu peak located at 932.1 eV. However, after a charge density of 0.5 C cm −2 , the peak of Cu disappeared. From these XPS results, we assumed that the Si˗O˗C was fabricated on the Cu substrate partially at a charge density of 0.3 C cm −2 . Afterward, the Si˗O˗C might be growing and covering on the Cu substrate after 0.5 C cm −2 .

Results and Discussion
Surface observation of Si˗O˗C deposited at 1.0 C cm ˗2 was investigated by SEM measurement and shown in Fig. 3a. The Si˗O˗C was synthesized on a surface˗treated Cu substrate with a low level of electricity, forming a Si˗O˗C thin film. Therefore, the rough surface of the Cu substrate might be reflected through Si˗O˗C composite, resulting in rough surface morphological characteristics. The EDS measurement of Si (yellow), O (green), C (red), and Cu (purple) are shown in Figs. 3b-3f. Also, Figure S1 illustrated the SEM˗EDS results of Si˗O˗C fabricated at a charge density of 0.3 and 0.5 C cm −2 . Those results revealed that all elements were deposited on Cu substrate homogenously. This homogeneous dispersity of Si˗O˗C formed by galvanostatic electrodeposition is one of the main reasons for the realization of the good cycle performances of Si˗O˗C anode because the O and C could reduce internal stress during Si volume changing by lithiation/de˗lithiation. 20,21,23 From the EDS analysis, it was confirmed  that the Si˗O˗C deposited at the low charge density below 2.0 C cm −2 also has homogeneous dispersity of all elements.
For the detailed chemical analysis of Si˗O˗C, in˗depth chemical profiles of Si˗O˗C prepared at a charge density of 1.0 C cm −2 were carried out by GDOES. For the GDOES measurement, the Si˗O˗C was cleaned by dimethyl carbonate in an Ar˗filled glove box and then dried in the vacuum chamber overnight. A different type of transfer vessel was used to prevent oxidization of Si˗O˗C composite. Figure 4 shows the result of GDOES analysis. The region from the surface to around 0.17 μm shows deposited elements such as Si, O, and C from Si˗O˗C composite. It is revealed that the mass ratio of Si is around 50˗60% of the entire of Si˗O˗C, implying that Si is the main element in the Si˗O˗C fabricated by electrodeposition as shown in the EDS measurement. Also, we confirmed that there is around 16˗20% of O in the Si˗O˗C compound. As described above, for the prevention of oxidization of Si˗O˗C, we used a transfer vessel for the GDOES analysis. Thus, the presence of O is not driven from the air or oxidized To further measure the thickness of Si˗O˗C, laser scanning confocal microscopy was conducted using Si˗O˗C prepared at a charge density of 1.0 C cm −2 . Before the investigation, the sample was washed with dimethyl carbonate and dried in the dry room for overnight to remove the solvent. As seen in Fig. 5a, the boundary region between Si˗O˗C and Cu substrate was selected to measure the difference of height between the top of Si˗O˗C and Cu substrate. Figures 5b and 5c represent the plane view and side view of Si˗O˗C fabricated on to Cu substrate. The average thicknesses of Si˗O˗C/Cu and Cu substrate are 0.30 and 0.12 μm, respectively. From the laser scanning confocal microscopy, the thickness of Si˗O˗C fabricated at a charge density of 1.0 C cm ˗2 was approximately 0.17 μm, and this is in good agreement with the GDOES analysis.
In our previous report, electrochemical behaviors of Si˗O˗C fabricated at a charge density of 2.0 C cm ˗2 were examined by cyclic voltammetry (CV). However, electrochemical reactions of Si˗O˗C fabricated at a lower charge density below 2.0 C cm ˗2 have not been reported. For that reason, the electrochemical behaviors of Si˗O˗C prepared at a charge density of 1.0 C cm ˗2 were tested by CV using a coin type half˗cell (Fig. 6). In the CV curve tested during the 1 st cycle, there is a large cathodic peak which was detected at around 0.1 V, which disappears after the 1 st cycle. This is revealed as the conversion reaction (SiO x → SiLi y ) and the formation of solid˗electrolyte interphase (SEI) on the Si˗O˗C surface. During the 2 nd cycle, lithiation and de˗lithiation peaks of Si˗O˗C were detected at around 0.09 V for lithiation and 0.32 V for de-lithiation. Those electrochemical behaviors are known as typical reactions of Si˗O˗C reported in the previous work. 23 From those results, it was confirmed that the Si˗O˗C fabricated at a charge density of 1.0 C cm ˗2 has a good lithium storage capability as an anode.
For high energy density of LIC, T. Aida et al. have compared a conventional LIC consisting of activated carbon and non-pre-lithiated anode and advanced LIC which has pre-lithiated anode. Through this study, it was confirmed that the overall potential can be extended from 3-4.5 V to 1.5-4.5 V vs Li/Li + by the low potential of pre-lithiated anode. Thus, it is possible to obtain the high energy density of LIC by pre-lithiation of the anode. 24 Hence, many methods such as chemical and electrochemical pre˗lithiation process have been widely studied to obtain optimized LIC full˗cell performance. [25][26][27] In this study, electrochemical lithiation was conducted at a 0.1 C˗rate with a voltage range  Figure 7a shows the charge curve of Si˗O˗C after pre˗lithiation of Si˗O˗C. The charge capacity of Si˗O˗C is 7580 mA h g −1 , and this value is higher than the theoretical capacity of Si. This high charge capacity during the 1 st cycle is attributed to the generation of SEI on the Si˗O˗C surface. Also, conversion reaction of SiO x is another reason for the huge charge capacity during the 1 st charge cycle, as mentioned in the CV analysis. For further investigation, a charge/discharge test was conducted using a half˗cell consisting of Si˗O˗C and lithium met al. Figure 7b demonstrates the charge/discharge curves of Si˗O˗C during the 1 st and 2 nd cycle. The irreversible capacity disappeared after the 1 st cycle. From these results, we confirmed that the irreversible capacity driven from additional reactions between Si˗O˗C with Li + ions, such as the conversion reaction and formation of SEI disappeared after the 1 st charge/discharge cycle. Also, this finding implies that the pre˗lithiation process of Si˗O˗C is a necessary step to realize the optimization of LIC performance.
In general, porous carbon such as activated carbon has been largely used as an electrode material for energy storage devices. However, in this study, CNT is used as a cathode material because it is easy to obtain a thin layer by electrochemical deposition technique to control the thickness. For that reason, the CNT thin layer fabricated by EPD is largely used for supercapacitors and LIC. 28,29 For the full˗cell configuration, CNT film was fabricated by EPD. The detailed information about sample preparation, materials and electrochemical characteristic of CNT film is described in supporting information (in Fig. S3˗S5). Before the LIC full˗cell test, optimization of capacity optimizing of cathode and anode was conducted. Figures 8a and 8b show the charge/discharge profiles of CNT cathode and Si˗O˗C anode during the 1 st cycle. The CNT cathode shows a discharge areal capacity of 22 μA h cm −2 during the 1 st cycle with good cyclability for 60 cycles. In addition, the Si˗O˗C anode delivered an areal capacity of 29 μA h cm −2 . Although there is a huge capacity reduction from the 1 st to 10 th cycle, it shows a good cyclability after the 10 th cycle. This good cyclability of both electrodes suggested that electrochemical deposition techniques can offer a good way to fabricate a high rate performance electrode for LIC application. As we described above, there is a capacity reduction from the 1 st to 10 th cycle, which is attributed to the SEI generation on the anode surface and conversion reaction (SiO x → Li x Si y /Li 2 O). For that reason, the capacity balance was controlled by discharge of areal capacity of the cathode (21 μA h cm −2 ) and anode (25 μA h cm −2 ) measured at the 10 th cycle. Based on half˗cell studies of Si˗O˗C and CNT film, the capacity ratio of anode and cathode for LIC construction was optimized as 1.2:1.0. The charge/discharge performance of LIC was investigated at a voltage range from 2.8 to 4.2 V and a current of 100 mA g (of AC) −1 . Figure 8c shows the cyclability of the LIC full˗cell for 1000 cycles. The CNT/Si˗O˗C LIC full˗cell demonstrated a good capacity retention of 95% for 1000 cycles with an outstanding coulombic efficiency of 98%. This result implies that the LIC consisting of Si˗O˗C and CNT has a good C˗rate capability. To examine the rate˗performance of LIC, the charge/discharge test of LIC was carried out with various C˗rates from 0.1 to 180 C. Figure 8d shows the rate˗performance of LIC depending on different C˗rate values. The capacity retention of LIC is 95.3, 94.8, 94.3 and 49.6% at 50, 100, 140, and 180 C˗rate. The good rate˗performance of LIC is attributed to the good cyclability of the Si˗O˗C anode. The Si˗O˗C synthesized by electrodeposition could be employed as a promising candidate for anode material of high˗rate performance LIC systems.

Conclusions
We investigated the electrochemical behaviors of Si˗O˗C as a high˗rate anode for LIC. The Si˗O˗C was fabricated at charge den-sities of 0.3, 0.5 and 1.0 C cm −2 . It was confirmed that the Si amounts can be controlled by charge density during Si˗O˗C electrodeposition by ICP analysis. Additionally, in the case of low charge density of 0.3 C cm −2 , the Si˗O˗C was partially fabricated on the surface of the substrate. Afterward, the Si˗O˗C covered the entire Cu substrate, forming the layer of Si˗O˗C which has a thickness of 0.17 μm. After cell optimizing between CNT film and Si˗O˗C, the LIC was assembled. The LIC exhibits good cyclability (capacity retention of 95% at the 1000 th cycle) and outstanding rate˗performance at different C˗rate conditions from 0.1 to 140 C (94.3% at a 140 C˗rate). The good electrochemical performances of the LIC full˗cell are related to the homogeneous dispersity of Si, O, and C in the Si˗O˗C which acts as a buffer to decrease internal stress during Si volume change. This study suggests a possible approach to employing Si˗based materials synthesized by electrodeposition as an anode for high rate˗performance LIC.