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Highly oriented graphene sponge electrode for ultrahigh energy density lithium ion hybrid capacitor Wook Ahn, Dong Un Lee, Ge Li, Kun Feng, Xiaolei Wang, Aiping Yu, Gregory Lui, and Zhongwei W. Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08298 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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ACS Applied Materials & Interfaces

Highly Oriented Graphene Sponge Electrode for Ultra-High Energy Density Lithium Ion Hybrid Capacitors Wook Ahn, Dong Un Lee, Ge Li, Kun Feng, Xiaolei Wang, Aiping Yu, Gregory Lui, Zhongwei Chen* Department of Chemical Engineering, University of Waterloo, 200 University Ave W. Waterloo, ON, N2L3G1, Canada. KEYWORDS Graphene sponge, oriented graphene, graphene electrode, high energy density lithium ion capacitor, lithium ion pre-doping.

ABSTRACT

Highly oriented rGO sponge (HOG) can be easily synthesized as an effective anode for application in high-capacity lithium-ion hybrid capacitors. X-ray diffraction and morphological analyses show that successfully exfoliated rGO sponge consists of on average 4.2 graphene sheets, maintaining its three dimensional structure with highly oriented morphology even after

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the thermal reduction procedure. Lithium ion hybrid capacitors (LIC) are fabricated in this study based on a unique cell configuration which completely eliminates the pre-doping process of lithium ions. The full-cell LIC consisting of AC/HOG-Li configuration has resulted in remarkable high energy densities of 231.7 and 131.9 Wh kg-1 obtained at 57 W kg-1 and 2.8 kW kg-1. This excellent performance is attributed to the lithium ion diffusivity related to the intercalation reaction of AC/HOG-Li which is 3.6 times higher that of AC/CG-Li. This unique cell design and configuration of LIC presented in this study using HOG as an effective anode is an unprecedented example of performance enhancement and improved energy density of LIC through succesful increase in cell operation voltage window.

Introduction Rising environmental concerns due to increasingly heavier energy-dependent life styles and activities of mankind are leading to growing interests in novel energy storage system developments to produce clean energies by harnessing renewable energy through photovoltaic and wind power generations.1-2 One primary contributor of environmental pollutions is gasolinebased vehicles which emit CO2 into the atmosphere.3-4 To address this, energy research has recently been focusing on the development of electrochemical energy conversion and storage devices such as high capacity lithium based batteries, metal-air batteries and high power supercapacitors.5-13 Simultaneously, technologies advancements in vehicles have resulted in onboard systems and equipment that require high performance energy supply.14-16 To fulfill the power requirements of vehicular propulsion, including auxiliary and safety systems of a vehicle, novel electrochemical energy systems such as hybrid capacitors must be developed, which combines the fundamental electric storage principles of a lithium ion rechargeable battery and an electric double layer capacitor.8, 17-19 As one type of hybrid capacitors, organic and ionic liquid


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electrolyte capacitor has been suggested20, which uses carbon-based negative electrodes, which significantly increases the energy density by allowing the carbon material that can store and release lithium ions to store and carry (hereinafter, in some cases, referred to as “pre-doping” or “pre-lithiation”) the lithium ions chemically or electrochemically in advance to lower the negative electrode potential.21-22 In the past decade a large number of researchers have sought to improve upon state-of-the-art lithium ion hybrid capacitors (LICs) for commercialization by utilizing positive electrode carbon materials from EDLC and lithium pre-doped negative electrode from battery applications such as Li1-xC6 of graphite or hard carbon.21, 23-24 However, current LICs require complicated device fabrication process, consisting of half-cell assembly for lithium pre-doping, cell disassembly, then re-assembly into a full-cell using the pre-doping electrode.17, 25-26 A coin-type battery is useful in terms of active material utilization and basically has a two-electrode structure. Additionally, the demand for lithium ion capacitors which can be used in high voltage window has gradually increased, but research and development on the capacitor is currently not actively being conducted. Therefore, it is urgent to develop high voltage operable lithium ion capacitors having high energy and power density with a stable ultralong lifetime.20, 27 Herein, we introduce, for the first time in the literature, the transformation kinetics of lithium diffusion mechanism in the full-cell configuration using highly oriented rGO sponge as the negative electrode for high energy and power density lithium ion capacitors. The cell is fabricated by directly attaching a piece of lithium metal on the backside of the anode as an effective lithium ion doping procedure to generate improved operating voltage windows for onestep activated full-cell LICs. Additionally, the activation procedure for pre-doping introduced here allows the fabrication of two-electrode cells without disassembling and re-assembling of the


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electrodes. Furthermore, the fabrication of two electrode coin-cell design makes previous artificial procedures obsolete, which is particularly interesting for significantly reducing costs associated with cell manufacturing. The following advantages of this research for the development of the state-of-the-art LICs are as follows: i) the highly oriented rGO sponge materials as an anode significantly improves the overall electrochemical properties including energy and power density of LICs; ii) the unique cell configuration design of two electrodes system has benefit to low manufacturing costs without any artificial Li pre-doping procedure; iii) the high mass loading of anode electrode leads to high energy density of LICs, which are particularly interesting for EVs and ESS applications.

Results and Discussions The detailed sample preparation is presented in the experimental section (Supporting Information), and we refer to the prepared materials as HOG (highly oriented rGO sponge), RAG (randomly aligned rGO sponge), CG (commercial graphite), AC (activated charcoal). The schematic illustration of the two electrodes lithium ion hybrid capacitor (LIC) system which does not require additional cell fabrication procedures for lithium pre-doping is presented in Figure 1a, and the ideal cell configuration of two electrode system with a complete consumption of Li metal for LICs is shown as Figure S1 in the Supporting Information. The direct lithium attachment on the back of the anode creates an internal short-circuit to induce pre-doping with lithium. This pre-lithiation process is innovative as it solves cell fabrication complexities associated with previously reported artificial procedures which generally consist of three stages: cell assembly for electrode lithiation, cell disassembly, and re-assembly into a full-cell. A previous work on fast and efficient pre-doping approach has confirmed that directly attaching


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lithium source on the anode for LIC fabrication is the fastest way to obtain a full-cell that works as well as electrochemically lithium ion pre-doped LIC.28 In addition to greatly simplified fabrication process, the LIC cell design in this report can be easily configured to fabricate larger multi-stacked cells via rapid lithium pre-doping as shown in Figure 1b. The effectiveness of the unique cell preparation presented in this study is demonstrated using high surface area HOG as the active electrode material as shown in Figure 1c. The XRD result presented in Figure 1d corresponds to the conventional patterns of graphite oxide (GO) and reduced graphene oxide (rGO). The peak observed with GO at the low angle corresponds to the (002) layer orientation in graphene oxides (d002 = 8.97o). The broad peak with a low intensity observed with HOG in the range of 20 to 30o is commonly known to be indicative of completely reduced GO. The structural evolution of HOG has been investigated by calculating the number of rGO layers based on the d୬ఒ

spacing and average crystallite size of the (002) plane obtained from the Bragg’s law (݀ = ଶ௦௜௡ఏ) ଴.଼ଽఒ

, and the Scherrer-Debye equation (‫ = ܿܮ‬ఉ௖௢௦ఏ). The calculated average number of rGO layers is 4.2, consistent with previously reported results29-30, indicating that the graphene sheets are completely exfoliated, and three-dimensional structure of rGO sponge is effectively maintained for enhanced electrochemical performance, which is demonstrated in the later section. Even after freeze drying and high temperature heat treatment, the graphene sheets in HOG are found to be well oriented, forming edge by edge connected porous structure as shown in Figure 1e. The TEM image of HOG sheets shown in Figure 1f reveals wrinkled edges as well as flattened areas. It is noted that the sudden freezing is reported to be effective for creating oriented and flattened planes of graphene oxide sheets as the frozen water inside the matrix of GO-water suspension helps to maintain the structure.31-34 The high-resolution TEM analysis of the HOG sheet has revealed that the sheet is composed to flattened thin graphene layers as shown in Figure 1g, and


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further investigation of HOG by high-resolution TEM analysis at two different locations reveals thicknesses of 1.63 and 2.59 nm, and the number of graphene layers of 6 and 8 as shown in Figures S2c, and S2d. This microscopic method to measure thickness and the number of layers of graphene has been shown to be effective in the previously reported literature..35 Furthermore, wrinkles in rGO sheets is known to adversely affect the electrical conductivity because graphene has a single 2D atomic layer (with sp2 carbon configuration and π–π complexation) in a honeycomb lattice, and electricity is conducted through the π–π bond.30, 36 Therefore, highly oriented and flattened rGO sponge (HOG) in this study presents high electrical conductivity for high power capacitor applications. Figure 2a shows typical Raman spectra of HOG showing peaks that correspond to D, G and 2D-band peaks of graphene.37-39 In particular, relatively high intensity of the D-band peak is ascribed to the defects of graphene layer (ID/IG=1.34). To further understand the physical structure of HOG for pre-lithiated LIC application, three dimensional Raman mapping of HOG has been obtained as shown in the Figure 2c, d followed by 100 points of selected area and indepth of 50 µm from surface to bulk (Figure 2b). The three dimensional mapping after two dimensional translation shows a gradient from white to blue which corresponds to high to low intensity of Raman response from HOG (Figure 2c, d). The horizontally alternating red and blue colors of the map indicate that the graphene sheets in HOG are indeed very well aligned. This observed graphene alignment leads to stable sponge formation as well as increased porosity and high electric conductivity for enhanced lithium storage during pre-lithiation and LIC operation. The 3D Raman mapping images obtained across the map and into the depth of HOG are presented in Figure S3, clearly illustrating the existence of voids surrounding a continuously oriented rGO sheets. The Raman spectrum obtained from 10 levels of HOG with each level


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being 5 µm in thickness (total depth of 50 µm) and consisting of 100 in-plane measurements as shown in Figure 2b, clearly indicates HOG very well maintains the oriented 3D structured even after grinding during electrode fabrication. In order to determine the orientation effect of rGO sponge on the electrochemical property, the freeze dried rGO sponge as a function of suspension concentration has been prepared. The morphological change and electrochemical property by testing half-cell are presented in Figure S4. The SEM image of RAG shows randomly aligned morphology with more exposed edges of rGO sheets, but suspensions high concentrations show edge by edge connected and highly oriented structure of rGO sheets (HOG). Furthermore, the initial reversible capacity with cycleability of HOG shows superior half-cell performance. It is well known that the reduced exposed edge of graphene sheets results in the irreversible capacity loss and high cycleability.4041

From this result, it is observed that higher concentration of suspension leads to more dense and

aligned structure of rGO sponge since they have less water content per dispersed unit volume of GO, which enhances the cooling rate, leading to slower growth rate of ice crystal. Cooling rates play a central role in determining the mean pore size, when the sample was frozen with a contact cooling plate. In brief, a larger temperature gradient leads to smaller macro-pores.42 To confirm the electrochemical working voltage windows of cathode and anode materials, half-cell experiments have been carried out versus the lithium reference electrode (Figure S5). We refer to the electrochemical full-cells of LIC as AC/CG-Li (full-cell consists of activated charcoal (AC) positive electrode and commercial graphite (CG)-Li metal directly attachment as the negative electrode), AC/HOG-Li (full-cell composed of activated charcoal (AC) as the positive electrode and rGO sponge (HOG)-Li metal directly attached). The performance of fullcell lithium ion hybrid capacitor fabricated based on the two electrode design consisting of


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cathode and anode with a lithium metal directly attached is evaluated. The full-cell LICs have been activated by charge-discharge cycling to allow diffusion of lithium ions into the anode as shown in Figure 3. The activation procedure was carried out by charging-discharging LIC in the voltage range of 2~4 V at the current density of 5 mA g-1 for five cycles in order to induce the lithium metal to fully diffuse into the anode. Following the activation, the actual full-cell cycling tests have been conducted within the voltage window of 1.5-4.2 V. As shown in Figure 3a and 3c, AC/CG-Li cell demonstrates a typical charge plateau of 3.7 V, which is also observed in the AC/HOG-Li cell. However, in contrast to the AC/CG-Li cell, the specific two plateaus observed with AC/HOG-Li is due to the SEI film formation (please refer to the Figure S5), which includes the reaction between residual oxygen functional groups of rGO sheets and lithium ions.43-45 Furthermore, AC/CG-Li and AC/HOG-Li show different trends during 1st and 2nd activation cycles, which can be understood based on voltage plateaus observed during half-cell activation of these electrodes as shown in Figures S5c, and S5d. AC/HOG-Li shows much longer discharge plateau due to the side reaction that occurs between Li ions and oxygen functional groups, as well as the formation of SEI, whereas AC/CG-Li shows significantly shorter 1st discharge plateau due to only the formation of SEI, and a very stable 2nd discharge plateau. These differences in the nature of activation of these electrodes therefore leads to different trends observed during full-cell activation as well. This confirms that the pre-doped full-cells by onestep activation procedure are prepared successfully for electrochemical evaluation as LICs based on the results of the half-cell tests. The cycling of both LICs at 10 mA g-1 presented in the Figure 3b, d shows the AC/HOG-Li cell performs about 10 % superior capacity and durable cycle life after 100 cycles. The superior initial cell performance of AC/HOG-Li is due to higher electric conductivity of HOG, which minimizes the internal resistance during electrochemical


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reactions. It is important to note that the capacity of the cells are limited by the capacity of cathode since the P/N ratio was not adjusted to 1:1, which results in the cathodes having relatively lower capacity than the anode for both AC/CG-Li and AC/HOG-Li cells. The P/N ratio of the electrodes has been determined based on capacities of both cathode and anode obtained from half-cell testing. HOG during half-cell showed a high voltage recovery as charge-discharge cycles advanced, which led to the testing of full-cell LIC in the limited voltage range of 0.2-0.01 V to maintain a stable cell operation. In this voltage range, the resulting operable capacities of HOG and CG were doubled that of AC, which resulted in the P/N ratio of 2:1. To compare the resistance alternation between activation and cell operation processes, DCR is measured by calculating the cell resistance between the last point of rest and the starting point of discharge procedure, and presented in Table S2. Both full-cells of AC/CG-Li and AC/HOG-Li show decreased resistances as the activation procedure advances, and the resistances of AC/HOG-Li during the activation and cell operation process are lower than those of AC/CG-Li. In addition to the demonstration of cyclability, the rate capability of pre-lithiated full-cell LICs is tested as shown in the Figure 4. Under varying rates, the specific capacities of AC/CG-Li and AC/HOG-Li are found to be 62.5 and 73.1 mAh g-1 at 20 mA g-1, and 43.6 and 43.1 mAh g-1 at 1000 mA g-1, and 60.8 and 70.6 mAh g-1 at back at 20 mA g-1, respectively, based on the mass loading of activated charcoal (AC) in the order of testing sequence. The specific capacity obtained with AC/HOG-Li is significantly higher compared to those obtained with AC/CG-Li at low tested current densities, even though the specific capacities of both cells become similar at higher tested current densities. From this result, it is concluded that the electrochemical properties mainly related to the high power performance LICs depends on not only cathode materials from EDLC, but also anode materials from LIB. Figure 4d shows cycleability at the


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current density of 20 mA g-1 after rate capability test, showing stable cycle property even after severe electrochemical testing. The energy density and the power density of AC/HOG-Li and AC/CG-Li cells with the P/N ratio of 2:1 are calculated based on gravimetric capacities of the cathode and the anode and presented in Ragone plot as shown in Figure 5a. The obtained energy densities of AC/HOG-Li and AC/CG-Li cells are 231.7 and 196.8 Wh kg-1 at 57 W kg-1, and 131.9 and 138.8 Wh kg-1 at 2.8 kW kg-1, respectively. The energy and power densities obtained in this study with AC/HOGLi are significantly higher than those obtained with graphene-based composites in an asymmetric capacitor which reports only 258 Wh kg-1 (based on cathode loading)46, as well as another work which reports 55 Wh kg-1 at 52 W kg-1 and 27 Wh kg-1 at 2.9 W kg-1.47 The superior performance obtained with AC/HOG-Li is attributed to the unique two electrode cell configuration prepared without a pre-lithiation process which allowed the lithium ion capacitor to be tested in the increased operable potential windows of 2.0 ~ 4.0 V (∆V: 2) and 1.5 ~ 4.2 V (∆V: 2.7). The energy and power densities with higher operating potential range of this unique cell configuration provides almost 1.8 times higher energy storage than that of previous lithium ion capacitors with lower operable potential window based on the specific energy obtained from the following equation: ଵ

‫ = ܧ‬ଶ ‫ ܸܥ‬ଶ

(1)

where, C refers to the specific capacitance of the cell, and V is voltage window of the cell (∆V). The cycle durability test of AC/HOG-Li cell has been performed in the voltage range of 2.0-4.2 V for the verification of stable cycleability, and presented in Figure 5b. For the cycle durability test, 108 Wh kg-1 has been obtained based on the mass of both cathode and anode at the power of 1.4 kW kg-1 with the capacity retention of 84.2 % after 103 cycles. From this result,


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the unique two electrode cell configuration system for pre-lithiation is observed to definitely effective for solving complicated manufacturing issues and obtaining stable LIC performance. In order to investigate the effectiveness of unique cell configuration of the two electrode system for LIC, the electrochemical behavior of cathode (AC) and anode (reference CG) was carried out individually as a function of charge and discharge time during the activation and cell operation procedure. A three-electrode cell system has been employed consisting of cathode/separator/anode-Li, and lithium metal as the reference electrode. In contrast to previously shown two electrode system, the amount of lithium required for a complete consumption during the internal short-circuit process has been calculated and loaded onto the anode (backside of graphite anode). As seen in the Figure S6a, a considerable voltage gaps are observed between the reference lithium and cathode (401 mV), and the reference lithium and anode (390 mV) at the point of fully discharged state during the activation process. This observed voltage discrepancy is most likely due to the incomplete lithium diffusion into the anode during the initial stage of the activation. After activation, however, the voltage gap observed at the cathode (224 mV) and the anode (202 mV) versus the reference lithium reduces, likely due to the stable LIC operation upon complete lithiation after the activation procedure (Figure S6b). It is very important to note that fully pre-doped lithium ions affect the cell operation voltage, thus making this two electrode system a unique cell configuration for LICs. To further understand the process of pre-doping of lithium ions, Galvanostatic Intermittent Titration Technique (GITT)46-47 has been employed to investigate the evolution of lithium diffusivity as a function of cell potential as shown in Figure 6a, b. Based on the data obtained by GITT, the lithium diffusivity of the electrodes has been calculated using Weppner and Huggins derived expression as shown below:


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‫ܦ‬௅௜ =

ସ୐మ ௱ாೞ ଶ గఛ

ቂ௱ா ቃ

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(2)

೟

where L refers to the electrode thickness, ߬ refers to interval time of the current pulse (600 s), and ∆Es is the steady-state voltage change, due to the current pulse and ∆Et is the voltage change during the constant current pulse, eliminating the iR drop.48 Figure 6c shows the lithium diffusivities as a function of cell potential of AC/CG-Li and AC/HOG-Li during the charge process, exhibiting similar tendency of lithium ion diffusion state. The lithium ion diffusivities of AC/CG-Li and AC/HOG-Li have been measured at three distinct points during charging to investigate the rate determining step of lithium ion hybrid capacitor; first at the starting point of 1.5 V, next at 3.0 V corresponding to the ion exchange occurrence step of Liା and PF଺ ି , then at the ending point of 4.2 V. The lithium diffusivities measured at these three points are plotted in the Figure S7 and summarized in Table 1. Higher lithium diffusivities have been obtained at the ion exchanging state of 3.0 V than at the ground state due to the Liା ion adsorption into the pores of the cathode at this stage. In fact, during this ion exchange step, AC/HOG-Li has demonstrated relatively higher potential compared to AC/CG-Li. Since both cells used the same activated charcoal cathode, this result is indicative of the intercalation of lithium ions from the surface to bulk occurring at a relatively lower voltage for AC/HOG-Li than AC/CG-Li. At the fully charged state of 4.0 V, the lithium diffusivity of AC/HOG-Li is significantly higher (3.6 fold) than AC/CG-Li, most likely due to much higher electrical conductivity of rGO leading to more rapid kinetics of lithium ion intercalation into HOG than CG. Also at the fully charged state, the highest lithium ion diffusivity for each cell is demonstrated since Liା ions and PF଺ ି counter ions are much readily transferred from the surface to the bulk of the anode, and absorbed into the cathode, respectively.


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Based on the results obtained from the GITT analysis, a possible model for the mechanism of rate determining step is suggested as shown in Figure 6d. Upon starting of the ion exchange, the lithium ions accumulate on the surface of the anode which is why after 2.65 and 3.28 V for CG and HOG, respectively, low lithium ion diffusivities are observed due to slow transformation kinetics from surface SEI to the bulk of anode. However, as lithium ions are further inserted into the inter-layers of the anode from the surface, the lithium ion diffusivity increases, due to the increased lithium ion mobility. These observations based on the magnitude of lithium ion diffusivity suggest that the lithium ion insertion is relatively slower than the inter-layer lithium ion diffusion step. This means that the performance related to kinetics of lithium ions in LICs is dependent on the anode, which is why superior results are obtained with high surface area, and highly conductive HOG making it a very promising active anode material for high power LIC applications. To investigate the actual electrochemical reactions of LICs, plots of differential capacity (dQ/dV) vs. potential (V) are reproduced from the charge-discharge profiles obtained in the voltage window of 2.0 to 4.0 V, and from 1.5 to 4.2 V for activation procedure (pre-lithiation procedure) and cell operation (after pre-lithiation), respectively. It is well known that dQ/dV analysis is one of the commonly used methods for verifying the real response of oxidationreduction step in an electrochemical cell during charge-discharge process.49 The reproduced dQ/dV curves of AC/CG-Li cell presented in Figure S8a shows a typical rectangular profile during the activation procedure which means the potential window of 2.0 to 4.0 V is stable for the cell operation, however, the dQ/dV profile shows relatively high oxidation profile with an inclined line beyond 4.0 V as shown in the result of the cell operating from 1.5 to 4.2 V. This result indicates that the electrochemical reaction is carried out at a high net voltage due to the Li


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ion intercalation reaction at the anode. AC/HOG-Li cell as shown in Figure S8b has broadened oxidation-reduction profile during both activation and practical cell operation. Similar to the aforementioned AC/CG-Li cell, the inclining peak is observed beyond 4.0 V due to the Li ion intercalation reaction. In contrast to the result of AC/CG-Li, however, AC/HOG-Li shows a bow tie shape profile, and the inflection points observed with AC/HOG-Li at both oxidation and reduction at 3.1 V are clear indications of the starting point of the ion exchange (Li ion starts to intercalate-deintercalate during charge-discharge) at the surface of HOG, and this result is in accordance with the reaction mechanism aforementioned above based on the GITT result. Figure S8 (c) illustrates Nyquist plots according to the equivalent circuit (inset image) obtained from the LICs of AC/CG-Li and AC/HOG-Li before and after the activation procedure (prelithiation). The measured ac-impedance spectrum of AC/HOG-Li shows lower charge-transfer (Rct) resistance compared to AC/CG-Li both before and after the activation procedure, which indicates that HOG has higher electrical conductivity than CG, which highlights the advantages of HOG as an effective active material for high power Li ion capacitors.

Conclusions In summary, highly oriented rGO sponge (HOG) is prepared by a facile synthesis method which allows the formation of three dimensional porous structure even after a high temperature thermal reduction process and pulverization of sponge to powder. In addition, a uniquely designed two electrode lithium ion capacitors are fabricated by attaching a lithium metal directly onto HOG anode to create an internal short-circuit, thereby pre-doping lithium without further cell disassembly and re-assembly. The full-cell LIC made with HOG exhibits excellent performance with high specific initial capacity of 71.9 mAh g-1 and a stable cycle durability of


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100 cycles at 20 mA g-1. The AC/HOG-Li cell demonstrates excellent energy densities, resulting in 231.7 and 131.9 Wh kg-1 obtained at 57 W kg-1 and 2.8 kW kg-1. Furthermore, the cycle durability result obtained in the voltage range of 2.0-4.2 V has resulted in108 Wh kg-1 based on the mass of both cathode and anode electrode at the power of 1.4 kW kg-1 with the capacity retention of 84.2 % after 103 cycles. The rate determining step analysis for lithium diffusion kinetics as a function of potential has revealed that HOG is superior compared to graphite. Additionally, the lithium diffusion reaction mechanism for the rate determining step of LIC has been investigated based on the resulted obtained from the GITT analysis from which a superior lithium diffusivity was demonstrated by HOG. Based on the electrochemical performance, HOG presents itself as a highly promising anode material for high power lithium ion hybrid capacitors, and the unique cell of directly attaching a lithium metal on the anode presented in this study offers a convenient one-step activation process without any further cell preparation for prelithiation.

FIGURES


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(a)

(b)

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(e)

(f)

(c)

(d) (g)

Figure 1. (a) Schematic diagrams of the LIC full-cell configuration composed of the activated charcoal as a cathode and negative electrode consisting of anode and lithium by directly attachment. (b) Effective stack cell configuration of two electrodes systethism LIC by direct lithium metal attached on the one side of anode. Each electrode material is casted on Cu and Al mesh current collector for high efficiency of lithium pre-doping by activation procedure. (c) Optical image of synthesized HOG which maintaining robust structure, and coin-type LIC fullcell consisting of two electrodes configuration. (d) XRD patterns corresponding to graphite oxide and HOG. The calculated average graphene layer using Scherrer-Debye equation is 4.2 layers. The morphology analysis of active materials for lithium ion hybrid capacitor: (e) highly oriented HOG; (f) TEM and (g) high resolution TEM images of HOG.


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(a)

(b)

(d)

(c)

Figure 2. Raman mapping analysis of HOG; (a) the representative Raman peak of HOG; (b) consisting of 100 in-plane measurement points of selected area; (c, d) shows 3D and 2D mapping chemical intensity images of HOG.


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(a)

(b)

(c)

(d)

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Figure 3. The performance comparison of LIC full-cell assembled by two electrodes system employing AC as the cathode, CG (a, b) and HOG (c, d) as the anode materials. Both full-cells are prepared by directly attaching lithium metal onto the anode to induce pre-doping. The results show activation processes of both cells for lithium pre-doping.


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(a)

(b)

(c)

(d)

Figure 4. (a~c) The rate capability of AC/CG-Li and AC/HOG-Li full-cells at current densities from 20~1000 mA g-1, and its comparison result. (d) The cycleability of prepared full-cells after rate capability test at a current density of 20 mA g-1.


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(a)

(b)

Figure 5. (a) Ragone plot of the AC/HOG-Li and AC/CG-Li cells; (b) cycle durability result of AC/HOG-Li cell by using unique two electrodes cell configuration without pre-lithiation fabrication.


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(a)

(b)

(c)

(d)

Figure 6. Galvanostatic Intermittent Titration Technique (GITT) curve vs. time of (a) AC/CG-Li and (b) AC/HOG-Li full-cells. The duration of the charge and discharge pulses have been calculated based on a current density of 10 mA g-1. (c) The lithium diffusivities of the AC/CG-Li and AC/HOG-Li full-cells as a function of the cell potential, which were determined by GITT during charge advanced; (d) Proposed model of transformation kinetics in depth of lithium diffusion followed by GITT measurement results; SEI film formation on the anode material affects the lithium diffusivity mainly related to the lithium insertion reaction from surface to inter-layer of anode materials.


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Table 1. Lithium ion diffusivities of AC/CG-Li and AC/HOG-Li obtained at three distinct charge potentials; 1.5 V, lithium ion diffusion domain from pores of cathode to electrolyte; 3.0 V, cation and anion exchange domain; lithium ion diffusion domain from surface to interlayer of anode materials.

Electrochemical Charge Potentials Sample 1.5 V

3V

4.2 V

AC/CG-Li

1.42 x 10-6

1.89 x 10-6

4.12 x 10-6

AC/HOG-Li

1.64 x 10-6

2.15 x 10-6

1.49 x 10-5

2

DLi (cm /sec)


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ASSOCIATED CONTENT Detailed preparation of materials and cell fabrication, electrochemical characterizations (half-cell test of activated charcoal, commercial graphite and rGO), SEM, Raman spectra, XPS results, and additional electrochemical properties such as GITT (Galvanostatic Intermittent Titration Technique), Rate capability, dQ/dV analysis and EIS are presented in the Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions Z. C. directed the research. Z. C. and W. A. designed the research. W. A., G. L. and K. F. conducted preparation of electrodes. W. A., A. U. and G. L. conducted Raman spectra mapping characterization and discussed on the result. W. A. and D. U. L. conducted all physical characterizations and discussion. W. A., K. F. and X. W. conducted electrochemical testing and discussion. W. A. and D. U. L. wrote the paper. All authors extensively discussed the results of the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This research was supported by NewTech Power Inc. and the Natural Sciences and Engineering Research Council of Canada (NSERC) through grants to Z.C. and the University of Waterloo. ABBREVIATIONS HOG, highly oriented rGO sponge; RAG, randomly aligned rGO sponge; CG, commercial graphite; AC, activated charcoal; AC/G-Li, full-cell consists of activated charcoal cathode and graphite-Li metal directly attachment; AC/rGO-Li, full-cell composed of activated charcoal as a cathode and rGO-Li metal directly attachment. REFERENCES (1) Li, N.; Wang, Y.; Tang, D.; Zhou, H., Integrating a Photocatalyst into a Hybrid Lithium– Sulfur Battery for Direct Storage of Solar Energy. Angew. Chem. 2015, 54, 9271–9274. (2) Zhao, H.; Wu, Q.; Hu, S.; Xu, H.; Rasmussen, C. N., Review of Energy Storage System for Wind Power Integration Support. Appl. Energy 2015, 137, 545–553. (3) Catenacci, M.; Verdolini, E.; Bosetti, V.; Fiorese, G., Going Electric: Expert Survey on the Future of Battery Technologies for Electric Vehicles. Energy Policy 2013, 61, 403–413. (4) Majeau-Bettez, G.; Hawkins, T. R.; Stromman, A. H., Life Cycle Environmental Assessment of Lithium–Ion and Nickel Metal Hydride Batteries for Plug–In Hybrid and Battery Electric Vehicles. Environ. Sci. Technol. 2011, 45, 4548–4554. (5) Ahn, D.; Xiao, X.; Li, Y.; Sachdev, A. K.; Park, H. W.; Yu, A.; Chen, Z., Applying Functionalized Carbon Nanotubes to Enhance Electrochemical Performances of Tin Oxide Composite Electrodes for Li–Ion Battery. J. Power Sources 2012, 212, 66–72. (6) Chen, Z.; Choi, J.-Y.; Wang, H.; Li, H.; Chen, Z., Highly Durable and Active NonPrecious Air Cathode Catalyst for Zinc Air Battery. J. Power Sources 2011, 196, 3673–3677. (7) Zhang, Z.; Wang, Q.; Zhao, C.; Min, S.; Qian, X., One-Step Hydrothermal Synthesis of 3D Petal-Like Co9S8/RGO/Ni3S2 Composite on Nickel Foam for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 4861―8486. (8) Zhou, G.; Paek, E.; Hwang, G. S.; Manthiram, A., Long–Life Li/Polysulphide Batteries with High Sulphur Loading Enabled by Lightweight Three-Dimensional Nitrogen/Sulphur– Codoped Graphene Sponge. Nat. Commun. 2015, 6, 7760. (9) Chen, W.; Xia, C.; Alshareef, H. N., One-Step Electrodeposited Nickel Cobalt Sulfide Nanosheet Arrays for High–Performance Asymmetric Supercapacitors. ACS Nano 2014, 8, 9531–9541. (10) Hassan, F. M.; Batmaz, R.; Li, J.; Wang, X.; Xiao, X.; Yu, A.; Chen, Z., Evidence of Covalent Synergy in Silicon–Sulfur–Graphene Yielding Highly Efficient and Long–Life Lithium–Ion Batteries. Nat. Commun. 2015, 6, 8597. (11) Park, H. W.; Lee, D. U.; Nazar, L. F.; Chen, Z., Oxygen Reduction Reaction Using MnO2 Nanotubes/Nitrogen-Doped Exfoliated Graphene Hybrid Catalyst for Li–O2 Battery Applications. J. Electrochem. Soc. 2012, 160, A344–A350.


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BRIEFS The most remarkable cell configuration without pre-lithiation procedure assembled with highly oriented 3D structured rGO sheet presents high power output in addition to high energy density and its retention, which are particularly interesting for high performance EV applications.

SYNOPSIS Highly oriented rGO sponge demonstrates excellent energy and power densities with long cycle life, resulting in 231.7 and 131.9 Wh kg-1 at 57 W kg-1 and 2.8 kW kg-1. Full-cell lithium ion hybrid capacitor is easily fabricated by directly attaching lithium metal on the backside of anode to induce lithium pre-doping without having to disassemble/re-assemble the cell. TOC


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Highly Oriented Graphene Sponge Electrode for ... - ACS Publications

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