Low Li Insertion Barrier Carbon for High Energy Efficient Lithium- Ion

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Low Li+ Insertion Barrier Carbon for High Energy Efficient Lithium-Ion Capacitor Wee Siang Vincent Lee, Xiaolei Huang, Teck Leong Tan, and Jun Min Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15473 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Materials & Interfaces

Low Li+ Insertion Barrier Carbon for High Energy Efficient LithiumIon Capacitor Wee Siang Vincent Leea, Xiaolei Huanga, Teck Leong Tanb, and Jun Min Xuea* a)

Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore. E-mail: [email protected] (J.M. Xue) b) Institute of High Performance Computing, A*STAR, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632 KEYWORDS: Energy efficiency, amorphous carbon, disordered carbon, voltage efficiency, practicality

ABSTRACT: Lithium ion capacitor (LIC) is an attractive energy storage device (ESD) which promises high energy density at moderate power density. However the key challenge in its design is the low energy efficient negative electrode which barred the realization of such research system in fulfilling the current ESD technological inadequacy due to its poor overall energy efficiency. Large voltage hysteresis is the main issue behind high energy density alloying/conversion-type materials which reduces the electrode energy efficiency. Insertion-type material though averted in most research due to the low capacity remains to be highly favourable in commercial application due to its lower voltage hysteresis. In order to further reduce voltage hysteresis and increase capacity, amorphous carbon with wider interlayer spacing has been demonstrated in the simulation result to significantly reduce Li+ insertion barrier. Hence by employing such amorphous carbon, together with disordered carbon positive electrode, a high energy efficient LIC with round tripe energy efficiency of 84.3 % with a maximum energy density of 133 Wh kg-1 at low power density of 210 W kg-1 can be achieved.

Introduction Energy efficiency has long been considered as one of the key parameters that influence the capital cost per cycle [1, 2], longevity of energy storage devices (ESD), and their operational safety. However, due to the shifting research focus towards development of high energy density ESD, energy efficiency is often less emphasized [3] as compared to other more recognized and heavily compared parameters. Such phenomenon has led to numerous reports of ESD with exceptional energy densities [4 – 10] which are unable to attain satisfactory energy efficiency, and hence significantly reduces the objective of ESD research. In order to realize fruitful ESD research, greater attention has to be focused on investigating and enhancing energy efficiency of ESD system. Lithium ion capacitor (LIC) is a wide potential window ESD which can synergistically deliver high energy density at moderate power density [11 – 13] due to the merger of lithium ion battery (LIB) negative electrode with supercapacitor positive electrode. While LIC remains as an exciting next-generation ESD, the fatal flaw in its fundamental design is the incorporation of low energy efficient LIB negative electrode (ca. 5 – 40 %, see S.I Figure S1), despite pairing with high energy efficient supercapacitor electrode. As such, numerous reported LIC systems which operate at high voltage window of more than 3 V attained energy efficiencies of 50 – 70 %. Hence, even though numerous successful LIC designs with high energy densities have been reported in past years, these devices are often impractical due to their lessthan-ideal energy efficiencies (i.e. ≥ 80 %). Therefore, this work, amongst few other studies, aims to realize the potential of LIC for next-generation technologies by designing and selecting appropriate LIB negative electrode.

The key challenge which results in low energy efficient LIB negative electrode is the low voltage efficiency of the electrode materials (since energy efficiency is proportional to voltage efficiency [14 – 16]). LIB negative electrodes, in particular alloying and conversion type materials, exhibit significant voltage hysteresis [17 – 19] (a resultant of a set of overpotentials due to mass/charge transfer process, chemical process, and etc. [16, 20]), which drastically lowers the electrode energy efficiency. The large voltage hysteresis hence renders the high energy density alloying/conversion type materials impractical as LIB negative electrode due to their lower voltage efficiencies. Ironically, insertion-type graphite which is averted in most research due to its low capacity 372 mAh g-1, remains to be the most promising LIB negative electrode [21, 22] due to its high energy efficiency. Insertion-type graphite typically has a significantly smaller voltage hysteresis as compared to high capacity alloying/conversion-type materials due to (1) its minimal chemical reaction (e.g. nucleation, phase transfer, and etc.) [14, 23], (2) its more efficient mass/charge transfer mechanism [24, 25], and (3) smaller structural expansion during lithiation/de-lithiation process. However, in addition to the low capacity, graphite possesses inherent tradeoffs such as low intercalation efficiency at high current rate [26] and phase transition during lithiation/de-lithiation [27, 28] which inevitably increase the electrode overpotential. In particular, based on the simulation result, significant Li+ insertion barrier has to be overcome in graphite during lithiation process which inevitability increase strain overpotential which ultimately increases voltage hysteresis. Hence, there is an urgent need to reduce this voltage hysteresis by reducing Li+ insertion barrier of insertion-type materials. Amorphous carbon is an attractive insertion-type LIB negative electrode material owing to (1) its higher specific capaci-

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ty (ca. 740 mAh g-1, Li2C6) as compared to graphite [29, 30], (2) its greater resistance to structural changes during ion intercalation process [31]. Most importantly, based on the simulation result, it was discovered that Li+ insertion barrier for amorphous carbon can be significantly reduced due to the wider interlayer spacing (ca. 0.47 nm) which is beneficial towards reducing both insertion overpotential and strain overpotential, and hence improving overall voltage efficiency of the negative electrode. In order to achieve a high overall LIC energy efficiency, high energy efficient positive electrode has to be employed as well. Since, unlike the negative electrode which is protected by the passivation of solid electrolyte interface (SEI), positive electrode is more prone to electrolyte degradation as it is unprotected from the electrolyte [32]. This results in unavoidable co-insertion of solvated ions into the interlayer spacing. The effect of solvated ions co-insertion is apparent at high voltage whereby the trapped solvated ions can be oxidized into gaseous product such as CO2 [11, 33], which the latter can exfoliate and pulverise the electrode structure. As shown in the previous work, disordered carbon in oppose to graphitic carbon could reduce the extent of entrapped solvated ions and hence reducing the intensity of electrolyte decomposition [34]. As such, disordered carbon is recommended as LIC positive electrode so as to improve the columbic efficiency of the electrode at wide voltage range. Herein, to the best of knowledge, this work is one of the first few studies to consider the LIC energy efficiency during the designing process. With the aid of simulation model, high performing, yet high energy efficient LIC was designed with amorphous carbon negative electrode and disordered carbon positive electrode in this work due to the lowered Li+ insertion barrier of amorphous carbon. The resultant 3.5 V LIC possesses both high energy and power density while at the same able to operate with a high round trip energy efficiency of 84.3 % at high current density of 1 A g-1. The designed system was able to deliver a maximum energy density of 133 Wh kg-1 at low power density of 210 W kg1 . Even at high power density of 11200 W kg-1, the device was able to deliver a respectable energy density of 42 Wh kg-1. Based on the previously reported high voltage window LIC (≥ 3.5 V), highest energy efficiency of ca. 70 % was reported with poor energy density of ca. 80 Wh kg-1, while highest energy density ca. 220 Wh kg-1 was reported at poor energy efficiency of ca. 55%. As compared to those previously reported works which either have high energy density or high energy efficiency (mutually exclusive due to abovementioned challenge for LIC negative electrode), the LIC designed in this work was able to attain good balance between energy efficiency of 84.3 % and energy density of 133 Wh kg-1, which positions it as a more appealing and practical LIC. In addition to high energy efficiency, high energy and power density, the devised LIC was able to achieve a good cyclic performance of 81.8 % capacitance retention after cycling for 5000 cycles at 5 A g-1. Thus, this work shows the possible harmonious co-existence of energy density and energy efficiency which serves as a guideline for developing practical high performing LIC so as to move beyond the experimental stage. In order to further improve the energy efficiency of LIC, it would require the efforts from cell engineer, on top of the proposed materials design.

Materials and Experimental procedures Materials: α-D-Glucose (anhydrous, 96%), sodium dodecyl sulfate (≥ 98.5 %), graphite (Darco, 100 mesh), and urea (anhydrous, 99%) were purchased from Sigma-Aldrich. Ethanol (absolute) and methanol (absolute) were also used in the experiment as solvents. Deionised water was used in all reaction. The materials were used without any further purification.

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Synthesis of amorphous carbon (AMC): In a typical synthesis, 5 g of α-D-Glucose and 25 mg of sodium dodecyl sulfate were dissolved in 50 ml of D.I water by vortexing and bath-sonicating for 10 mins. The resultant clear solution was later transferred into a Telfon-lined hydrothermal bomb and into a hydrothermal oven for 5 hours at 180 oC. After hydrothermal process, black slurry was collected and washed with D.I water and ethanol repeatedly. The hydrothermal product was later left to dry in the electric oven overnight. The dried product was eventually calcinated at 800 oC for 2 hours at a ramping rate of 10 oC min-1 in N2 atmosphere. The calcinated product was denoted as AMC and was used for electrochemical testing without any further purification. Synthesis of porous disordered carbon sheet (PdCS): In a typical synthesis, 5 g of urea, 0.5 g of α-D-Glucose, and 25 mg sodium dodecyl sulfate were dissolved in 3 ml D.I water, 9 ml ethanol, and 3 ml methanol. The mixture was bath-sonicated for 10 mins in order to obtain a clear solution. The solution was later magnetically stirred in a hot silicone oil bath at 75 oC for 1 hour to obtain white solid. The white solid was later left to dry in an electric oven (80 oC) overnight to obtain yellowish sweet-smelling crystals. The yellowish crystals was later collected and calcinated at 350 oC for 4 hours with ramping rate of 5 oC min-1, which was further calcinated at 950 oC for 10 hours with ramping rate of 5 oC min-1 in N2 environment. The final obtained sample was denoted as PdCS, and was used for electrochemical testing without any washing. Electrochemical fabrication of half-cell, and LIC (AMC//PdCS). All half-cell devices studied for their material performance in this work were assembled using the standard industrial method, following the recommend method to best characterizing the performance of the device. To prepare the negative electrode half-cell, AMC was mixed with polytetrafluoroethylene (PTFE), carbon black in a ratio of 8:1:1 with N-methyl-2pyrrolidone (NMP). The mixture was hand-grinded for at least 15 minutes to obtain a black slurry. The slurry was later coated onto copper foil which served as a current collector using the thick film coater. After heating at 80 oC overnight, the sheet was pressed and punched into 1.2 cm diameter electrode with mass loading of about 0.7 mg. To prepare the positive electrode half-cell, PdCS was mixed with polytetrafluoroethylene (PTFE), carbon black in a ratio of 8:1:1 with N-methyl-2-pyrrolidone (NMP). The mixture was hand-grinded for at least 15 minutes to obtain a black slurry. The slurry was later coated onto aluminum foil which served as a current collector using the thick film coater. After heating at 80 oC overnight, the sheet was pressed and punched into 1.2 cm diameter electrode with mass loading of about 1.2 mg. Individual halfcell measurement, in the form of coin-cell, was conducted for both the AMC electrode and PdCS electrode, with lithium plate as the counter and reference, in 1 M LiPF6 dissolved in 1:1:1 (volume) mixture of ethylene carbonate/diethyl carbonate/dimethyl carbonate. LIC was assembled with the pre-lithiated AMC electrode (by cycling for 50 cycles at 1 A g-1) and PdCS electrode. Electrochemical measurements. All the electrochemical test were performed at room temperature. Cyclic voltammetry (CV) and galvanostatic charge-discharge were studied using BioLogic Science Instruments VMP 3. The specific capacitance (C: mAh g1 , and F: F g-1) was calculated using the following formula, C = I ∆t / 3.6 (1) F = I ∆t / ∆V (2) Where I is the current density (A g-1), ∆t is the discharge time (s), and ∆V is the potential difference (V)


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Figure 1. (a) Schematic showing the diffusion pathway of Li (purple atom) in graphite. At low concentration, Li adsorbs most stably at the hexagonal site. The transition state occurs with Li located above the bridge site connecting two carbon atoms. .(b) Calculated energy barriers along the diffusion pathway for selected interlayer spacing of graphite (c) Diffusion barrier vs. interlayer spacing of graphite

Based on the total mass of the active materials from the positive and negative electrode, energy density (E) and power density (P) were calculated based on the following formula

∙ / 3.6

(3)

P = 3600 E / ∆t (4) Where V is the voltage range (V), I is the current density (A g-1), t is the time (s), t1 is the time which the cell is fully discharged, t2 is the time which the cell is fully charged, ∆t is the discharge time (s). All measurements were conducted at room temperature (25 o C). Energy efficiency is calculated based on the discharging area and charging area from galvanostatic charge/discharge with resolution of 0.1 second. Material Characterizations. SEM images of AMC and PdCS were recorded on ZEISS SEM Supra 40 (5kV). SEM samples were prepared by dripping the sample solutions onto silicon substrate. TEM was done on JEOL-3010 (300kV acceleration voltage) for both AMC and PdCS. TEM samples were prepared by dripping the sample solutions onto lacey carbon grid. Powder Xray diffraction (XRD) pattern was measured by a powder diffractometer (Bruker D8 Advanced Diffractometer System) with Cu Kα (1.5418 A) source. Powder Raman spectrometry was conducted for AMC, and PdCS on Horiba MicroRaman HR Evolution System using an Argon laser beam with an excitation wavelength of 514.5 nm. BET measurement for AMC, and PdCS was performed with Micromeritics ASAP 2020 surface area and porosity analyser.

toxic, and environmentally friendly precursors. Amorphous carbon (AMC) was prepared by pyrolysing dewatered glucose (via hydrothermal at 180 oC) at 800 oC in nitrogen environment, while disordered porous carbon sheets (PdCS) was prepared via simple pyrolysis of dewatered glucose/urea at 950 oC. In this work, LIC was assembled with AMC as negative electrode and PdCS as positive electrode, and such assembly was denoted as AMC//PdCS. From first-principles calculations, it is demonstrated that the diffusion barrier of Li+ decreases with the interlayer distance of graphite (Figure 1). In normal (crystalline) graphite, the interlayer distance is small (calculated d = 3.67 Å) which translates to a high diffusion barrier of 0.62 eV (Figure 1b). The high energy barrier leads to slow diffusion of Li+ in graphite, which reduces the transfer kinetics process and hence results in lower energy efficiency due to the increased voltage hysteresis. Increasing the interlayer distance, d, decreases the diffusion barrier as shown in Figure 1c. For amorphous carbon with d ~ 4.7 Å, the diffusion barrier is decreased to 0.07 eV, thus explaining why a higher energy efficiency could be achieved by replacing normal graphite with expanded ones. Structure-wise, Li becomes asymmetrically located between the carbon layers when d > 4.4 Å (see S.I Figure S2). Hence based on the simulation result, energy barrier can be greatly reduced when interlayer spacing is at 0.47 nm (greater than graphite) which hints the importance of amorphous carbon in reducing Li+ insertion barrier (strain overpotential and transfer kinetic overpotential).

Material Characterization and Electrochemical performance of AMC Negative Electrode

Results and Discussion Material synthesis of both negative and positive electrodes were based on simple pyrolysis route which involve low-costing, non-

Morphology of AMC was firstly studied with scanning electron microscopy (SEM) and the image was shown in Figure 2a. Necking of particles can be clearly observed with the SEM image (Figure 2a) and such necking among particles can provide a continu-



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Figure 2. Material characterization of AMC. a) SEM image, b) low-resolution TEM (inset showing the SAED), c) high resolution TEM showing the interlayer spacing (inset showing magnified portion of interlayer spacing), d) Raman spectroscopy of AMC from 1000 to 3000 cm-1 (inset showing the magnified portion between 2200 – 3000 for determination of 2D band), and e) high resolution C1s XPS spectrum of AMC (inset showing O1s XPS spectrum).

ous conduction channel which is beneficial for charge transport. Furthermore the observed smooth carbon surface is advantageous for the formation of a more efficient solid electrolyte interface (SEI) layer [35] which can provide greater stability during lithiation/de-lithiation. The necking (interconnection) of nanoparticles is further confirmed when the sample was investigated under low resolution transmission electron microscopy (TEM) as shown in Figure 2b. The selected area electron diffraction (SAED) pattern of AMC revealed diffused rings which suggests the disordered state of the synthesized carbon. Under close examination with high resolution TEM, as shown in Figure 2c, relatively welloriented expanded interlayer spacing with defects can be clearly observed which hints that the synthesized carbon possesses a disordered structure with slight graphitic character with the interlayer spacing shown in Figure 2c inset. The wider interlayer spacing was estimated to be ca. 0.4 nm based on x-ray diffraction (XRD) as shown in Figure S3, which is larger than the 0.34 nm graphitic interlayer spacing. Such widened interlayer spacing, which is typical of disordered carbon, is beneficial for (1) the reduction of Li+ insertion barrier energy and hence reducing strain overpotential (increased structure stability), and (2) the enhanced removal of any co-intercalated solvated ions out of the structure (due to the breakdown of SEI layer). In order to further verify the disorder degree of the synthesized AMC, Raman spectroscopy was conducted as shown in Figure 2d. The Raman spectrum of AMC revealed 2 modes, the D peak around 1345 cm-1, and G peak around 1583 cm-1 due to the breathing modes of rings and the relative motion of sp2 carbon atoms respectively. ID/IG of AMC was calculated to be ca. 1.01 based on the peak height ratio, which may suggest the disordered state of the sample. The broadening of D band as shown in Figure 2d indicates a distribution of clusters with different dimensions, and the presence of ring orders other than six [36], which further suggests the disordered nature

of AMC. At higher Raman shift (Figure 2d inset), small signal can observed at ca. 2900 cm-1 which indicates the presence of insignificant graphitic ordering [37]. Thus, based on the collective results from TEM and Raman spectrum, AMC possesses disordered nature with slight graphitic character. X-ray photoelectron spectroscopy (XPS) was conducted for AMC to determine the oxygen functionalities in the sample. AMC shows a typical reduced graphene oxide high resolution C1s XPS spectrum (77 % sp2 and 13 % sp3 C) which is due to the ion bombardment on the sample during the XPS measurement, resulting in the formation of more stable sp2 C from the metastable sp3 C [38]. Based on the high resolution C1s XPS spectrum, AMC contained small proportion of C-OH (6.9 %) and C-O (3.4 %) functional groups which were further supported by the high resolution O1s XPS spectrum (Figure 2e inset). Thus, the low oxygen atomic concentration (ca. 10 %) in the synthesized AMC as determined by XPS is essential for constructing an efficient conduction channel, and also a lower extent of solvated ions adsorption on the plane. The half-cell assembly of AMC was studied using Li plate as both counter and reference electrode, and its electrochemical performance (conducted in ambient condition) was summarized in Figure 3. The sample was cycled with a potential window of 0.01 to 3 V and its cyclic voltammetry (CV) curves were shown in Figure 3a. AMC revealed cathodic peak which is characteristic of Li+ insertion into disordered carbon which is in contrast to the prominent redox peaks in graphite CV curve. The cathodic peak in the first cycle (ca. 0.2 V) is attributed to the decomposition of electrolyte to form SEI layer on the negative electrode. Rate performance of AMC was studied by increasing current densities from 0.1 to 5 A g-1 as shown in Figure 3b. AMC was able to deliver a specific capacity of 579, 469, 390, 327, 276, and 202 mAh g-1 at current density of 0.1, 0.2, 0.5, 1, 2, and 5 A g-1 respectively. With a 50


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ACS Applied Materials & Interfaces cyclic stability of AMC negative electrode was investigated by deep-cycling at 1 A g-1 for 200 cycles as shown in Figure 3e. The negative electrode demonstrates a good cyclability as a 121 % increment in specific capacity of 427 mAh g-1 was achieved from an initial capacity of 352 mAh g-1 after 200 cycles due to electrochemical activation. In addition, coulombic efficiency of AMC was maintained at ca. 100 % for all cycles which further indicates its suitability as high energy efficient LIC negative electrode. The higher energy efficiency and higher cyclic stability of AMC are a resultant of the following design considerations; (1) interconnected nanoparticles with partial graphitic character and low oxygen functionalities to provide efficient conduction channels, and hence improve the charge/transport, (2) larger interlayer spacing which aid in an increment of 5 % energy efficiency due to the lowered Li+ insertion barrier (strain overpotential) and in the structural stability during lithiation/de-lithiation process, and (3) smooth nanoparticles surface can increase the structural stability due to the formation of more efficient SEI layer which enhance the cyclic stability and coulombic efficiency.

Figure 3. Electrochemical performance of AMC negative electrode halfcell with a potential window of 0.01 to 3 V. a) First 3 CV curves of AMC conducted at a scan rate of 1 mV s-1, b) rate performance of AMC with various current densities (0.1 to 5 A g-1), c) first 3 galvanostatic charge/discharge profile at 0.1 A g-1, d) energy efficiency with different current densities (0.5, 1, 2, and 5 A g-1) (inset showing voltage hysteresis), and e) cyclic stability test conducted at 1 A g-1for 200 cycles, and its corresponding coulombic efficiencies.

times increment in current density (0.1 to 5 A g-1), an initial capacity retention of 34.9 % can be achieved which indicates the moderate kinetic capability of the negative electrode due to its interconnected conduction pathway. The electrode was also able to return to a specific capacity of 347 mAh g-1 at current density of 1 A g-1 which indicates excellent reversibility. As disordered carbon is well-known to exhibit large initial irreversibility due to the formation of SEI layer, first 3 galvanostatic charge/discharge profiles of AMC were presented in Figure 3c. An initial irreversible capacity loss of 859 mAh g-1 was calculated which can be attributed to the formation of SEI, and such SEI formation is sensitive to the surface area [39, 40] or the so-called active surface area [41] of the sample. Based on Brunauer–Emmett–Teller (BET) N2 adsorption result, AMC possessed a BET surface area of 243 m2 g-1 (Figure S4) which is mainly responsible for the large capacity loss due to SEI layer formation. However the subsequent cycles gave a coulombic efficiency of ca. 100 % with a reversible discharge capacity of 600 mAh g-1 after 3 charge/discharge cycles. Two distinct segments can be observed in the galvanostatic charge/discharge profiles which are namely the sloping region at higher potential region (≥ 0.9 V), and the plateau region at lower potential region (≤ 0.9 V) [42]. The sloping region is due to the intercalation of Li+ between turbostratically disordered graphene sheets, while the plateau region is due to the formation of small Li metal clusters inside the hard carbon nanopores [43]. Energy efficiency of AMC negative electrode was studied by varying the current density as shown in Figure 3d. AMC was able to achieve an energy efficiency of 54.7, 51.9, 47.8, and 38.2 % at 0.5, 1, 2, and 5 A g-1, as compared to the corresponding graphite energy efficiency of 43.8, 46.7, 42.1, and 31.4 % as shown in Figure S5. Galvanostatic charge/discharge profile was also presented as voltage hysteresis graph as shown in Figure 3d inset. Both AMC and graphite negative electrodes face Li+ ion mobility limitation (mass transport limitation) at high current density. Thus, as observed from the individual energy efficiency at 1 – 5 A g-1, a constant energy efficiency mismatch of ca. 5 % may suggest the beneficial effect of larger interlayer spacing on the Li+ insertion barrier and reducing strain overpotential. The

Material Characterisation and Electrochemical Performance of PdCS Positive Electrode Morphology of the synthesized PdCS was investigated under SEM as shown in Figure 4a. The SEM image revealed highly interconnected sheets with highly accessible pores, which is advantageous for electron/mass transportation and electrolyte accessibility. TEM was also conducted, and the low resolution TEM image shown in Figure 4b indicates a high density of nanosheets interconnecting one another to form the porous structure, which further supports the ultra-thin sheet-like structure of PdCS as observed in the SEM. The weak diffusive rings in the SAED pattern (Figure 4b inset) suggests disordered nature of PdCS. Raman spectroscopy was also conducted to further verify the disorder nature of the sample. As shown in Figure 4d, weak D and weak G peaks can be observed at 1340 cm-1 and 1581 cm-1 in the Raman spectrum. Even though ID/IG does not give a clear indication of the disorder degree of the sample, a low 0.98 ratio may hint the disordered state of the sample [36]. In addition, the broadening of D band is indicative of increasing disorder in the structure [36]. At higher Raman shift (Figure 4d inset), no visible peak can be observed which further indicates the disordered nature of PdCS. As it is commonly consented that a low oxygen functionalities in electrode is important to reduce the oxidation of electrolyte [44, 45], dehydration of organic precursor is one of the strategies to decrease the number of functional groups [46]. Thus to investigate the oxygen functionalities in PdCS so as to study its stability as supercapacitor electrode, XPS was conducted. High resolution C1s spectrum is shown in Figure 4e which revealed the presence of C-OH and C=O functionalities which is in good agreement with the high resolution O1s spectrum (Figure 4e inset). Based on the XPS result, PdCS possesses a low oxygen functionalities of 2.8 % which is important for minimizing electrolyte oxidation. Another important aspect of supercapacitor electrode is to possess high surface area with nanopores < 2nm so as to increase the energy density of the system [47, 48]. Based on BET result (Figure S6), PdCS possessed a BET surface area of 616 m2 g-1, with a typical Type 1 isotherms with small H4 hysteresis (Figure S6a), which suggests that the sample consists of micropores with exposed surface almost entirely inside the microspores with majority of pore distribution smaller than 2 nm (Figure S6b). Electrochemical performance of PdCS was investigated by assembling in a half-cell configuration with lithium plate as both the counter and reference electrode. Figure 5a shows the CV curve of PdCS conducted at 25 mV s-1 between potential window of 1.5 to 4.5 V. The CV curve is close-to-rectangular in shape which shows



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Figure 4. Material characterisation of PdCS. a) SEM image, b) low-resolution TEM (inset showing the SAED), c) high resolution TEM showing the interlayer spacing (inset showing magnified portion of interlayer spacing), d) Raman spectroscopy of PdCS from 1000 to 3000 cm-1 (inset showing the magnified portion between 2200 – 3000 for determination of 2D band), and e) high resolution C1s XPS spectrum of PdCS (inset showing O1s XPS spectrum).

that the synthesized PdCS displayed a close approximately to that of electric double layer capacitor. An S-value was conducted for PdCS as shown in Figure S7, and based on the obtained S-value [49], the synthesized carbon remained to be stable at high potential (i.e. S < 0.1) which indicates its suitability for high voltage positive electrode. In order to study for its rate performance, PdCS half-cell was tested under increasing current densities (0.1 to 20 A g-1) as shown in Figure 5b. PdCS was able to deliver a specific capacitance of 157, 158, 152, 145, 138, 125, 121, and 108 F g-1 at current density of 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 A g-1 respectively. An excellent rate kinetic is observed for PdCS as even with a 200 times increase in current density, a respectable initial capacitance retention of 68.8 % is achieved. Such good rate kinetic demonstrated by PdCS could stem from its ultra-thin, overlapping carbon sheets which firstly reduces the diffusion length, and secondly the formation of a continuous conductive channels due to the carbon sheets overlapping. Based on the relatively linear charge/discharge profile shown in Figure 5c, this further suggests the EDLC mechanism contribution from PdCS. A high energy efficiency of 87.8 % was achieved when the PdCS was charged/discharged at high current density of 1 A g-1. At higher current densities (e.g. 5 A g-1 and higher), increasing overpotential was observed (as the coulombic efficiency remains to be ca. 100 %) which is mainly due to the limited mass transport. Thus this observation demonstrates the limitation of the Li+ and solvated ions mobility in the electrolyte which lowers the voltage efficiency of the cell at high current, and hence reduces the overall energy efficiency. At high current density of 2, 5, 10, and 20 A g-1, energy efficiency of 86.7, 86.5, 84.3, and 79.4 % were attained by PdCS half-cell as shown in Figure 5d. Galvanostatic charge/discharge profile was also presented as voltage hysteresis graph as shown in Figure 5d inset. As supercapacitor electrode has to ultimately operate at high current densities, such high energy efficiencies as attainted by PdCS further demonstrates its suit-

ability as a high performing and energy efficient supercapacitor electrode. Finally, as a golden standard for supercapacitor electrode, a good cyclic stability must be achieved. To investigate PdCS cyclic stability, the half-cell was deep cycled at 10 A g-1 for 100000 cycles as shown in Figure 5e. The half-cell was able to achieve a remarkable capacity retention of 97.2 %, which is an attractive property for long lasting supercapacitor application. In addition, the columbic efficiency of PdCS remained close to 100 % throughout the cycling test, which indicates minimum electrolyte degradation process. Thus, based on the collective results, PdCS is suitable as LIC positive electrode due to the following reasons: (1) High capacitance due to its high surface area, (2) high rate performance due to the enhanced kinetic by constructing efficient conduction channels, (3) high attainable energy efficiency at high current density whereby supercapacitor should operate at, and (4) long cycle life and close to 100 % coulombic efficiency which are both essential for ensure the longevity of capacitor.

Electrochemical performance of LIC AMC//PdCS While SEI layer formation on AMC is unavoidable, the loss of Li+ from the electrolyte can be minimized by pre-lithiating the negative electrode with a sacrificial lithium plate, prior to the normal operation of LIC. Thus, before assembling the full LIC comprising of AMC negative electrode and PdCS positive electrode (AMC//PdCS), AMC was firstly pre-lithiated by cycling the negative electrode with Li plate in half-cell configuration. In order to balance the charge between the negative and positive electrode, pre-lithiated AMC was coupled with PdCS with a mass ratio of 2:3. The assembled AMC//PdCS was later investigated for its electrochemical performance which is summarized in Figure 6. CV was conducted for AMC//PdCS between 0.5 to 4 V at a scan rate of 25 mV s-1 as presented in Figure 6a. The CV curve shows close approximation to the ideal EDLC mechanism which is crucial for achieving high overall energy efficiency. The rate perfor-


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mance of the assembled LIC at various current densities was summarized in Figure 6b. Based on the total mass of the active materials from both electrodes, the device was able to deliver a specific capacitances of 65, 63, 60, 57, 54, 47, and 39 F g-1 at current density of 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g-1 respectively. The device was able to show excellent rate performance due to enhanced kinetic, especially at low current density, as even with a 100 times increment in current density, a 60 % capacitance is still achievable. Such superior rate capability can be a resultant of excellent kinetic matching of the negative and positive electrodes. After deep cycling at high current density, the device was able to return to a specific capacitance of 46 F g-1 at 5 A g-1 which indicates its high reversibility. In order to examine the energy efficiency of the system, galvanostatic charge/discharge profiles at various current densities (0.1 to 1 A g1 ) are presented in Figure 6c. It is commonly known that with higher current density, voltage efficiency of the device will drop drastically [14] due to the increasing overpotential for mass/charge transport. However based on the designed AMC//PdCS, round trip energy efficiencies of 81, 84, 85, and 84 % are obtained with increasing current density of 0.1,0.2, 0.5, 1 A g-1 respectively. Even at high current density of 10 A g-1 a respectable energy efficiency of 51 % is achievable. The lower energy efficiency at high current density demonstrates the limitation set by the Li+/ion mobility in the electrolyte. Such high energy efficiencies at high current densities demonstrate the importance in designing LIC with efficient conduction channels. Energy and power density of the highly energy efficient LIC were calculated (based on total mass of active materials from both negative and positive electrodes) and were summarized in a Ragone plot shown in Figure 6d. At power density higher than LIB (210 W kg-1), the device (based on the total mass of the active materials for both electrodes) was able to deliver a high energy density of 133 Wh kg-1. Even at high power density of 11200 W kg-1 (in the region of supercapacitor), the device was still able to deliver a respectable 42 Wh kg-1, which is higher than most supercapacitors. While higher performing LICs with higher energy and power density

Figure 5 Electrochemical performance of PdCS positive electrode with a potential window of 1.5 to 4.5 V. a) CV curve of PdCS at scan rate of 25 mV s-1, b) rate performance of PdCS at various current densities (0.1 to 20 A g-1), c) galvanostatic charge/discharge profiles at high current densities (1 to 20 A g-1), d) energy efficiencies at different current densities (inset showing voltage hysteresis), and e) cyclic stability of PdCS conducted at 10 A g-1 for 10000 cycles.

have been reported in literature, these exceptional performances

Figure 6 Electrochemical performance of AMC//PdCS operating with voltage range of 3.5 V (between 0.5 to 4 V). a) CV scan of the full LIC at 25 mV s-1, b) rate performance of LIC with various current densities, c) galvanostatic charge/discharge profile at 1 A g-1 showing an energy efficiency of 87.3 %, d) Ragone plot (based on the total mass of the active materials of both negative and positive electrode), e) cyclic stability performance of LIC for 5000 cycles at 5 A g-1

are usually obtained at the expense of energy density (more details in S.I Figure S8). AMC//PdCS LIC even though was designed with low capacity insertion-type disordered carbon, the full cell performance is able to match with those which employ transition metal oxides, while attaining a higher energy efficiency. Cyclic stability of the assembled LIC was also investigated by deepcycling the device at high current density of 5 A g-1 for 5000 cycles. The hybrid AMC//PdCS was able to demonstrate a respectable cyclic stability whereby a 81.8 % capacitance retention was achieved after 5000 cycles. Thus, the devised LIC can operate for 5000 cycles (or a cycle life of ca. 13 years) before it comes impractical [50]. In order to further investigate the performance of the system, a 3 electrodes setup with charge balance between the negative and positive electrodes was employed to study the individual operation potential window of the negative and positive electrodes when the voltage range is between 0.5 – 4 V (Figure S9). Based on the 3 electrodes electrochemical test shown in Figure S9, to operate a full voltage range of 3.5 V (i.e. 0.5 – 4 V), AMC negative electrode will have to operate within a potential window of 0.3 – 1.4 V while PdCS positive electrode will have to operate within a potential window of 1.8 – 4.3 V. This motivates the further investigation of the respectively electrode energy efficiency within these utilized potential windows. As shown in Figure S10, when operated between 0.3 – 1.4 V, the energy efficiency of AMC negative electrode increases from a 51 % to 77 %. Similarly, PdCS positive electrode, when operated at 1.8 – 4.3 V, was able to achieve an enhanced energy efficiency of 91 %. Assuming a charge balance between negative and positive electrodes, an energy efficiency of 84 % for the full cell can be estimated by averaging the energy efficiencies of both negative and positive electrodes. Thus, energy efficiency of LIC is ultimately dependent on the individual efficiency of the electrodes, with the negative electrode being the main culprit for the low energy efficiency for LIC. While voltage range can directly influence energy density, it can, at the same time, indirectly influence the energy efficiency of LIC. A set of voltage range was examined for its energy efficiency and energy density as shown in Figure S11. When the voltage



range of AMC//PdCS was decreased to 3 V (i.e. 1 – 4 V), an energy efficiency of 87.0 % is attained. With even lower voltage range of 2.5 V (1.5 – 4 V), and 2.2 V (1.8 – 4 V), higher energy efficiencies of 91.7 and 92.0 % can be achieved respectively. However, even though energy efficiencies were improved through lowering of voltage range, the energy density of the device was sacrificed through this process. Thus this observation serves as a reality reminder that there is a dilemma between increasing energy density and improving energy efficiency.

* [email protected]

Funding Sources This work was supported by Singapore MOE Tier 1 funding R284–000–162–114 and National University of Singapore Strategic Fund R261509001646 & R261509001733.

REFERENCES References

Conclusion In view of the insignificant attention to the practicality aspect of the ESD (energy efficiency) as compared to more commonly adopted electrochemical parameters such as energy and power density, this work hopes to increase awareness in the importance of energy efficiency of the system before unrealistically improving the energy and power density. Thus, to improve the overall energy efficiency of the system, LIC was devised in this work with amorphous carbon AMC and disordered PdCS. As it is shown in this work, overall energy efficiency of the device is ultimately an average of individual energy efficiency of the negative and positive electrodes (assuming a 1:1 charge pairing). Thus it is important to improve the individual energy efficiency of each electrode, in particular LIC negative electrode which is usually the main culprit for the lower energy efficiency. To tackle the energy efficiency of negative electrode, the following strategies were proposed; amorphous carbon was introduced due to (1) wider interlayer spacing to reduce the Li+ insertion barrier and to increase the structural stability, (2) higher capacity insertion-type material as compared to graphite, (3) small graphitic character to ensure more efficient conduction channels, and (4) low oxygen functionalities to reduce the electrolyte degradation. On the other hand, disordered carbon was introduced as positive electrode due to (1) lower oxygen functionalities to reduce electrolyte degradation, (2) thin overlapping carbon sheets to promote an efficient conduction channels, and (3) high surface area with nanopores to increase the energy density. With these design considerations, the devised LIC was able to operate with voltage range of 3.5 V. The device was able to deliver a high energy density of 133 Wh kg-1 at 210 W kg-1 while achieving an high round trip energy efficiency of 84.3 % at high current density of 1 A g-1. In addition to the high energy density, power density, and high energy efficiency, the device was able to demonstrate a cyclic stability of 81.8 % capacitance retention after 5000 cycles at high current density of 5 A g-1. Thus, this work highlights the possible co-existence of high electrochemical performance and high energy efficiency when the abovementioned strategies were considered in the LIC design.

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Acknowledgments This work was supported by Singapore MOE Tier 1 funding R284–000–162–114 and National University of Singapore Strategic Fund R261509001646 & R261509001733. The author would also like to thank Dr. Tamie Loh Ai Jia for her invaluable and fruitful discussion

Supporting Information. Supporting Information Available: Energy efficiency definition, simulation details, 3 electrode setup, energy efficiencies of previously report LIB negative electrode and LIC, energy efficiency of the full cell (3 electrodes investigation)

AUTHOR INFORMATION Corresponding Author

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Low Li Insertion Barrier Carbon for High Energy Efficient Lithium- Ion

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