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sodium ion batteries
Journal of Power Sources 329 (2016) 412e421
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Bio-derived hierarchically macro-meso-micro porous carbon anode for lithium/sodium ion batteries Indu Elizabeth a, b, c, **, Bhanu Pratap Singh b, c, d, Sunil Trikha e, Sukumaran Gopukumar a, c, d, * a
CSIR-Central Electrochemical Research Institute, Karaikudi, India Physics and Engineering of Carbon, CSIR-National Physical Laboratory, New Delhi, India Academy of Scientific and Innovative Research, India d CSIR- Network Institutes for Solar Energy, India e EON Electric Ltd, Noida, India b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Prawn shells are converted into porous carbon through economically viable process. The derived carbon exhibits excellent capacity in both Li/Na ion batteries. NMR (Li6, Li7 & Na 23) and EPR studies on intercalated derived carbon carried out.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 22 August 2016 Accepted 25 August 2016
Nitrogen doped hierarchically porous carbon derived from prawn shells have been efficiently synthesized through a simple, economically viable and environmentally benign approach. The prawn shell derived carbon (PSC) has high inherent nitrogen content (5.3%) and possesses a unique porous structure with the co-existence of macro, meso and micropores which can afford facile storage and transport channels for both Li and Na ions. PSC is well characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Transmission electron Microscopy (TEM), High resolution TEM (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Electron Paramagnetic Resonance (EPR) and Solid state-Nuclear Magnetic Resonance (NMR) studies have been conducted on pristine PSC and Li/Na interacted PSC. PSC as anode for Lithium ion batteries (LIBs) delivers superior electrochemical reversible specific capacity (740 mAh g1 at 0.1 Ag-1 current density for 150 cycles) and high rate capability. When used as anode material for Sodium ion batteries (SIBs), PSC exhibits excellent reversible specific capacity of 325 mAh g1 at 0.1 Ag-1 for 200 cycles and rate capability of 107 mAh g1 at 2 Ag-1. Furthermore, this study demonstrates the employment of natural waste material as a potential anode for both LIB and SIB, which will definitively make a strike in the energy storage field. © 2016 Published by Elsevier B.V.
Keywords: Lithium ion batteries Sodium ion batteries Anode Bio carbon Specific capacity NMR
* Corresponding author. CSIR-Central Electrochemical Research Institute, Karaikudi, India. ** Corresponding author. CSIR-Central Electrochemical Research Institute, Karaikudi, India, CSIR-National Physical Laboratory, New Delhi, India E-mail addresses: [email protected] (I. Elizabeth), gopukumar@cecri. res.in (S. Gopukumar). http://dx.doi.org/10.1016/j.jpowsour.2016.08.106 0378-7753/© 2016 Published by Elsevier B.V.
I. Elizabeth et al. / Journal of Power Sources 329 (2016) 412e421
1. Introduction Lithium ion batteries (LIBs) have become the universal power source for most of the portable consumer electronics since they were launched in 1981. Currently, they are the technology of choice to develop electric vehicles and large scale storage of renewable energy because of their high energy density and high voltage. But these applications call for higher specific capacities and faster rate performance. A typical modern LIB cell consists of a cathode made from a lithium-intercalated layered oxide, a graphite anode and an organic electrolyte. The battery operates following a “rocking chair” concept in which the Liþ ions shuttle between the anode and cathode through the electrolyte during the charge/discharge cycles [1]. In spite of extensive research on electrode materials for LIB, still they are lacking in terms of rate capabilities, capacities and economic viability. Novel highly structured and cheap electrode materials have to be investigated for improved performances. In this context, if LIBs are to gain a significant place in the future, there is going to be an increasing demand for Li, resulting in concerns about the increasing cost and limited abundance of Li. Sodium ion battery (SIB) is one possible alternative to LIB owing to its low cost and higher natural abundance of sodium particularly when large scale applications are considered. The state of the art anode for LIB is graphite but exhibiting low capacities when used in SIBs as only a small amount of Na can intercalate into graphite [2]. Disordered carbon with porous morphology appears to be very promising as anode for SIBs. Many carbonaceous materials with various morphologies and structures like carbon nanotubes (CNTs) [3], hollow nanospheres [4], graphene [5] etc. have been investigated as anode materials for rechargeable batteries. However, the complex synthesis methods, expensive equipments and the use of toxic/harmful chemicals make it less attractive. Recently, carbonaceous material derived from biomass has been explored widely as anode material for LIBs [6e9]. Biomass derived carbon has the advantage of being a renewable, low cost source, and also can provide excellent porous 3D nanostructure which is of very particular interest for high lithiation/sodiation capacity and excellent cyclability in batteries [10]. N-doping of carbon is proven to improve the electrochemical performance, as it enhances the electrical conductivity and also provide more sites to increase the interaction between carbon and Li/Na during cycling. N-doped carbons are synthesized typically using various expensive, toxic N containing precursors following rigorous/complex experimental procedures. In this point, preparation of N-doped carbon from naturally occurring bio waste/ biomass is very appealing as it reduces the environmental pollution, lessens the usage of toxic chemicals and is very inexpensive both in terms of source cost and processing cost. Several studies have been reported recently on biomass derived anode material for LIB/SIBs like carbon material derived from human hair [11e13], ox horn [14], agricultural waste [15], peanut shells [16,17], wheat straw [18], banana peels [19], coconut shell [20], fish scales [21] and plant biomass [8,22,23]. In this work, we have synthesized a hierarchical macro-mesomicro porous, turbostractic, N-doped carbon material derived from marine Indian prawn shells. The Indian prawn (Fenneropenaeus indicus, formerly Penaeus indicus) [24], is one of the major commercial prawn species of the world. F. indicus is known by many common names around the world, including Indian white prawn, Tugela prawn, white prawn, banana prawn, Indian banana prawn and red leg banana prawn. About 10 10 to 10 11 tons of prawn shells are produced each year as natural waste. In addition, capture fisheries and aquaculture also contributes enormous quantity of prawn shells as waste side product [25,26]. Prawn/shrimp
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crustacean shells contains nitrogen containing polysaccharide, chitin (poly-b(1-4)-N-actetyl eD-glucosamine) and inorganic components mainly CaCO3. Hence, prawn shells are an inexpensive, easily available, abundant source of nitrogen containing biomass which can be put into use for a wide variety of applications. N edoped highly porous carbon has been synthesized from prawn shell by a very simple method, which is not only economical but also environmental friendly. Porous carbon derived shrimp/ prawn shells has been used for applications like supercapacitors [27,28] and catalyst for oxygen reduction reaction (ORR) [29] recently. We have evaluated carbon obtained from prawn shells as anode material for LIBs and SIBs and obtained excellent electrochemical performances. Herein, we have successfully converted a waste material to a wonder/wealth material for high end applications. To the best of our knowledge, this is the first time prawn shell derived carbon is used as electrode materials in both LIB and SIBs, which we believe will revolutionize the present energy scenario. 2. Experimental section All the chemicals used in the study were of analytical grade from Sigma-Aldrich and Merck and used as received without further purification. De-ionized (DI) water was prepared on site using Millipore-Q water system. The Indian prawn shells used in this study were collected from the local fish market (Karaikudi, India), a waste material. 2.1. Preparation of prawn shell derived porous carbon (PSC) The Indian prawn shells were washed thoroughly with water and dried in air. The material was heated at 300 C and grinded. The pre-carbonized material was immersed in 0.1 M NaOH solution for 24 h followed by heating at 750 C under inert argon atmosphere for 2 h with a heating ramp of 3 C min1. NaOH is used as an activation agent for creating porous structure by leaching out the acidic impurities. The obtained black material was washed extensively with 1 M HCl to remove the metal impurities like Ca, followed by washing with DI water till neutral pH was attained. The resulting carbon material was dried at 80 C under vacuum. The asfabricated prawn shell derived carbon material is designated as PSC hereafter. 2.2. Material characterization The surface morphology of the synthesized powder was examined using a FESEM (Model: KARL ZEISS) and HRTEM (Tecnai 20 G2(FEI). XPS of the synthesized powder has been performed using a MULTILAB 2000 (Thermo Scientific) photoelectron spectrometer and the spectra were recorded using an X-ray source (MgKa radiation 0e1253 eV). XRD have been carried out with a Bruker D-8 diffractometer equipped with a Cu Ka radiation (1.5406 Å) operated at 40 kV and 30 mA. Raman spectroscopy has been carried out at room temperature with Green LASER (excitation line 514 nm at 2.5 mW power) using Renishaw InVia Reflex Micro Raman spectrometer equipped with a CCD detector. EPR studies of PSC before and after interaction with Li and Na, was conducted using EPR spectrometer (Model: BRUKER EMX Plus) at microwave frequency of 9.863,940 GHz and 10 mW power at room temperature. 6Li, 7Li and 23Na NMR of PSC after intercalating with Li/Na was recorded using NMR instrument (BRUKER, 400 MHz). N2 sorption analysis was performed at 77.4 K using QUANTASORB equipped with surface area and pore size analyzer. Sample was degassed at 150 C for 24 h before analysis. BET surface area was determined according to BET model. The electrochemical measurements were carried out by
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fabricating 2032 coin type cells in an argon-filled glove box with Li/ Na metal as counter as well as reference electrodes, for LIB and SIB respectively. The working electrode slurry was prepared by using 80% PSC, 10% super-P carbon, and 10% PVDF (polyvinylidenefluoride) binder blended with NMP (N-methyl-2-pyrrolidone) solvent and coated over a Cu foil. The polypropylene separator was soaked in an electrolyte (LiPF6 in 1:1 EC: DEC for LIB and in house synthesized NaPF6 in EC/DEC for SIB) and sandwiched between the electrodes. The galvanostatic charge/discharge studies were carried out at different C-rates with the assembled coin cells using an automatic battery cycle life tester (WONATECH, Korea) between 0.01 and 3 V. Electrochemical impedance studies (EIS) were carried out using Frequency Response Analyzer (Biologic, USA) with an AC signal of 5 mV in the frequency range from 100 kHz to 5 mHz. Post-mortem of the cycled cells was done to study the structure intactness of the anode material. The anode samples were taken out from the cell and cleaned using the electrolyte solution inside the glove box and TEM studies were conducted. 3. Results and discussions The (002) region of XRD spectrum of PSC (Fig. 1a) exhibits a
superposition of a weak and broad amorphous carbon peak, particularly towards the lower angle range (around 2q ¼ 23 ) and a strong graphitic peak at 26.6 . Inset of Fig. 1a shows two distinct deconvoluted peaks. In addition, a weak (100) band is also observed at ~ 40 which also points to the presence of crystalline carbon or graphite like structure in PSC. These observations suggest that carbon derived from prawn shell has an intermediate structure between graphite and amorphous called turbostratic structure, in which the graphene layers randomly translate to each other [30,31]. The d002 spacing calculated using the Bragg equation from the angular location of the strong peak is 0.35 nm which matches with turbostratic structure [32,33]. Fig. 1b depicts the Raman spectrum for the synthesized PSC. Two peaks are noted at 1347 cm1 (D band) and 1594 cm1 (G band) corresponding to the structural defects and disorder induced in the graphene layers of carbon and the first order stretching vibration of sp2 carbon atoms in the two dimensional hexagonal lattice respectively [34]. The intensity ratio of D-band to G-band (ID/ IG) is 1.02 which indicates the presence of defects and imperfections in the structure. Partial replacement of carbon atom by nitrogen in PSC can also promote defects. A broad 2D is also seen in the Raman spectra at ~2800 cm1 implying layering of graphene
Fig. 1. (a) XRD pattern and (b) Raman spectra of PSC and (c) Nitrogen adsorption-desorption isotherm of PSC. Inset - the pore size distribution of PSC (magnified image provided in supporting document Fig. S5).
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sheets (turbostractic structure) [35]. N2 adsorption-desorption isotherm at 77.4 K (Fig. 1c) presents typical type IV shape with H2 hysteresis located at a relative pressure of 0.4e0.99, which is characteristic for meso-porous structure [36]. The specific surface area of PSC is 336 m2 g1. The comparatively lower BET surface area of PSC can reduce the unwanted irreversible side reactions and lessen the irreversible capacity loss [37]. The pore size distribution of PSC [inset of Fig. 1c] shows mainly meso-pores in the size range of 2e10 nm and also some meso pores with ~20 nm diameter. It is clearly visible that PSC is also composed of micro-pores of size <2 nm. Also, some macro pores of size around 50 nm are observed. The hierarchically porous nature of PSC is more clearly confirmed from the TEM and HRTEM studies. The uniqueness of PSC nanostructure was studied using SEM, FESEM, TEM and HRTEM. Fig. 2a depicts the SEM image of PSC from a macroscopic perspective, showing PSC consisting of nearly spherical particles of diameter around 1 mm. However, from the images taken under higher magnification, FESEM (Fig. 2b) and TEM (Fig. 2c and d) exhibit more complex carbon structure. Fig. 2b represents that the PSC particles are made up of graphene analogue carbon sheets which are randomly stacked. Highly porous graphene analogue sheets of size ~500 nm randomly arranged with some degree of graphitization are clearly visible in TEM images (Fig. 2c and d). Further examination under higher magnification, the PSC structure has been found to be more exciting. The TEM images (Fig. 3a, b and c) reveals that the graphene analogue carbon nano sheets are made up of clusters of meso porous, spherical carbon nano sheets (Meso-CNS) of size around 20 nm. Meso-CNS are
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arranged in such a way that there are macro pores between them which can act as ion buffering reservoirs and also help in better electrolyte penetration [38]. Interestingly, it is important to mention here that the presence of meso-pores in Meso-CNS offers the possibility of fast ion transfer by reducing the transportation path thereby favours the penetration of electrolyte to realize better performance of the electrode material [12] and provides ideal surfaces for the adsorption of Li/Na ions with a very small innerpore resistance [38e40]. A closer inspection of Meso-CNS using HRTEM (Fig. 3d, e and f) shows micro-pores and localized graphite layers, which are the typical for turbostratic carbon structure. The inset of Fig. 3e is the Selected Area Electron Diffraction (SAED) pattern which clearly shows some crystallinity due to turbostratic layering of carbon as confirmed from XRD. It is highly plausible that such a structure will provide additional intercalation sites for accommodation of Li/Na ions, resulting in the higher specific capacity. PSC with its unique, magnificent, hierarchical macro-meso-micro porous structure is expected to exhibit excellent electrochemical performance in both SIBs and LIBs. X-ray photoelectron spectroscopy (XPS) has been used to quantify the elemental composition of PSC. As depicted in Fig. 4 the XPS survey spectrum exhibits three major peaks at ~284, 400 and 532 eV corresponding to C1s, N1s and O1s respectively. The peak assignment and relative atomic percentages are tabulated in Table S1 (Supporting data). High resolution XPS analysis of N1s photoelectron envelope indicated the presence of structurally integrated pyridine (398.35 eV; area 36.2%) and pyrrolic N (400.04 eV; area 63.8%) [14]. In the table, it can be seen that the mass percentage of doped nitrogen is as high as 5.3%. The
Fig. 2. (a) SEM image of Prawn shell derived carbon (PSC) (b) FESEM image of PSC showing graphene analogue carbon sheets stacked (turbostratic structure) (c) and (d) TEM image of PSC showing porous graphene analogue sheets.
Fig. 3. (a), (b) and (c) TEM images of PSC. (d), (e) and (f) HRTEM images of PSC.
Fig. 4. (a) XPS survey spectrum (b) C1s, (c) N 1s and (d) O 1s spectra of PSC.
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electrochemical performance of PSC material is improved by the high N content as it enhances the electronic conductivity as well as formation of defects which can act as active Li/Na storage sites [14]. As shown in XPS, C1s spectrum (Fig. 4b) the most characteristic peak of carbon (sp2 C]C) is located at 284.8 eV and the remaining peaks are sp3 C (284.29 eV), CeN nitrile (285.50 eV), pyrolic carbon linked to nitrogen CeN/CeO (286.40 eV) and C]O (288.29 eV). The total C content in PSC is around 78%.The fractional percentage of various groups in C 1s spectra (Table S1) indicates that the PSC has large number of structural defects that can support rapid charge etransfer reactions and act as Li/Na insertion sites, thereby improving the electrochemical performance. The O1s spectrum indicates the presence of three types of oxygen bonds eC]O (530.73 eV), OeCeO (532.06 eV) and OeC]O (533.33 eV) [14]. The oxygen mainly comes from the thermally stable groups in carbon except for the absorbed oxygen or moisture on the carbon surface. The oxygen as doped heteroatom can play a positive role for Li storage by increasing defects, disorder, or local electron density around O atoms according to previous studies [41]. EPR technique is a useful tool for understanding the mechanism of reaction of alkali metals with carbon as it can detect the localized paramagnetic centres assigned to p aromatic radicals, which can react electrochemically with Li and Na [42e44]. The EPR spectra of pristine PSC are presented in Fig. 5. An isotropic signal with Lorentzian curve shape with g ¼ 2.0022 and line width of ~0.36 mT, which originates from localized paramagnetic centres is observed. The origin of these localized radicals can be attributed to the imperfect carbon structure, such as dangling bonds with unpaired electrons on the edges of small esize porous graphene sheets (as seen in TEM) [45].
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After electrochemical intercalation with Li and Na (Fig. 5b and c), the EPR signal coming from the original localized radicals are perturbed. Similar feature has been observed earlier in the case of petroleum cokes treated below 1000 C [43] and in surface modified Carbon Microbeads [46], wherein the reason is ascribed to the reaction of Li/Na with dangling bonds and hydrogen in the edge of graphene layers, resulting in the formation of covalent bonds, contributing to the disappearance of this signal [45]. Electrochemical reaction of PSC with Li/Na results (Fig. 5b and c) in broader EPR signals with one order of magnitude higher than that of original localized radicals with nearly same g value (about 2.0028). The signal can be considered as the combined contribution from localized paramagnetic centres with different local environments. For instance, localized paramagnetic centres can be formed due to Li/Na interaction with the edges of graphene layers or Li/Na intercalation between the turbostratic layers or Li/Na adsorption on the surface of small-size disordered graphene sheets [46]. 6Li and 7Li NMR studies of lithiated PSC (Supporting document Fig. S1) also shows the evidence of different environments of inserted Li. 23 Na NMR studies given in the supporting document (Fig. S2) also show the PSC is suitable for Na ion intercalation also. Comparatively lesser intensity of EPR signals from Na interacted PSC can be attributed to the lower amount of Na that can be accommodated in the PSC structure. The Dysonian asymmetry in the line shape (Table 1), characterized by high ratio of A/B, low field peak to high field peak amplitudes is observed in Li/Na interacted PSC [47]. This indicates an increased number of conduction electrons in the sample after Li/Na intercalation [48].
Fig. 5. EPR spectra at room temperature of (a) PSC (b) Li intercalated PSC and (c) Na intercalated PSC. (d) g-value of PSC and Li/Na intercalated PSC.
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Table 1 EPR parameters of PSC, Li interacted PSC and Na interacted PSC. Sample
Signal intensity I (a.u.)
EPR line width DHpp (mT)
Dysonian asymmetry (A/B)
PSC PSC/Li PSC/Na
4.8 162 91
0.36 1.34 1.25
~0.13 ~1.04 ~1.02
3.1. Electrochemical studies 2032 type coin cells were assembled to evaluate the electrochemical performance of PSC in LIBs. In order to understand the interfacial reaction between the electrode and electrolyte and investigate the transport kinetics involved in electrochemical performance Electrochemical Impedance Studies (EIS) measurements were conducted for the Li ion coin cell before and after cycling in the frequency range 100 kHz to 5 mHz. As depicted in Fig. 6a, the Nyquist plots of PSC consist of semicircle in the high and middle frequency regions and a straight line in the low frequency region. The matching equivalent Randles circuit has also been provided as inset in Fig. 6a. Here Rs is the solution resistance of the cell which includes resistance from the electrolyte; Cdl is the double layer capacitance; Rct is the charge transfer resistance; and Zw is the Warburg impedance related to the lithium diffusion in the electrode [49,50]. EIS spectra is fitted and the corresponding parameters were derived which is summed up in Table 2. The solution resistance Rs does not show any variation indicating the electrical stability of the electrolyte. The charge transfer resistance, Rct, decreases after cycling which indicates an
improvement of interfacial conductivity for fast charge transfer between the electrode and electrolyte. The decrease is because of the large irreversible Li insertion into the electrode during cycling owing to its surface area and porosity thereby improving the conductivity [51]. The value of double layer capacitance (Cdl) remains almost constant during the cycling implying formation of a stable SEI. The exchange current density (I0) can be calculated according to Equation (1).
I0 ¼ RT=nFRct
(1)
Where R is the gas constant, T is the absolute temperature, n is the number of transferred electrons and F is the Faraday constant. I0 is an important parameter influencing the rate of a charge transfer controlled electrochemical reaction [52]. Here, the increase in I0 with cycling indicates a favourable charge transfer process of Li ion insertion/extraction reaction. There is also decrease in the diffusion resistance, Zw, after cycling demonstrating that the structure of PSC is suitable for Li ion diffusion leading to superior electrochemical performance. The galvanostatic charge/discharge performance of PSC has
Fig. 6. (a) Impendence spectra of PSC before and after cycling. In the inset is equivalent Randles circuit. (b) Rate capability studies of PSC at different current densities. (c) Cycling capability of PSC at current densities of 0.1 Ag-1 and 0.75 Ag-1. (d) Galvanostatic charge and discharge profile of PSC for the 1st,10th and 100th cycle at 0.1 Ag1 and 1st cycle at 0.6 and 2 Ag-1 current densities in LIB.
I. Elizabeth et al. / Journal of Power Sources 329 (2016) 412e421 Table 2 EIS spectra fitting parameters of PSC anode in LIB. S.No
Parameters
Before cycling
After cycling
1 3 4 6 7
Rs (U) Rct(U) Zw (U) Cdl (F) I0 (mA cm2)
4.73 ± 0.274 145.1 ± 0.608 88.39 ± 0.301 3.56e-6±32e-9 0.177
4.70 ± 0.251 83.50 ± 0.918 55.03 ± 0.462 3.29e-6±44.4e-9 0.3079
been carried out at different current densities in the voltage range 0.01 Ve3 V such as shown in Fig. 6b for determining the rate tolerance of the material. When cycled at 0.1 Ag-1 current density the PSC electrode shows a high initial capacity of 1013 mAh g1, which is more than thrice the capacity of pure carbon signifying that there are other lithium storage ways in PSC other than conventional graphite intercalation. Although large irreversible capacity loss is observed in first two cycles, the capacity stabilizes after the second cycle to 740 mAh g1. The heavy capacity fade during the initial cycles can be attributed to the large amount of Li which is getting irreversibly trapped in pores of PSC and in the formation of SEI layer. In other words during the formation cycles (initial few cycles), the carbon electrode gets converted from its pristine state to an active Li storage host. The PSC electrode exhibits a reversible specific capacity of 740, 653, 436, 298 and 147 mAh g1 at 0.1, 0.2,0.8, 1.6 and 2 A g1 (5C) respectively (Fig. 6b). 1C rate is taken as 372 mA g1. After cycling at very high rate, the reversibility of the material was checked by cycling at current densities 1.2, 0.6 and 0.1 Ag-1. The initial reversible capacity (723 mAh g1) at 0.1 A g1 was recovered implying excellent rate capability of the material. Fig. 6d depicts the voltage
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vs. specific capacity profile of the electrode material at different rates. During the first charge cycle, at 0.1 A g1, plateau is observed around ~0.8 V corresponding to the SEI formation which is typical for carbonaceous material [3]. The appearance of low voltage plateau is mainly due to the filling of the pores by Li [53]. The appearance of plateau in the discharge profile around 0.25 V corresponds to the extraction of Li from PSC in the EC þ DEC electrolyte [54]. At high C rates the disappearance of the plateaus may be due to the high activation polarization developed while applying high current between the electrodes [51]. The life time performance of the PSC was tested by cycling the cell for 150 cycles at two different current rates namely 100 (0.25 C) and 750 (2 C) mA g1 as depicted in Fig. 6c. The electrode material shows a very stable specific capacity of 730 mAh g1 when cycled at 0.25 C with a coulombic efficiency of ~99.9% after the initial few cycles. At 2C rate, the PSC exhibits a specific capacity around 470 mAh g1, with excellent cycling stability up to 150 cycles, which is more than thrice the practical capacity of graphite. Owing to the developed meso/micro porosity and good electrical conductivity of PSC, it was evaluated as SIB anode by galvanostatic charge-discharge test in the voltage range 0.01 Ve3 V (Fig. 7). EIS spectra (Fig. 7a) shows a decrease in Rct resistance, from 420 U before cycling to 200 U after cycling for the investigated 200 cycles implying an increase in interfacial conductivity upon cycling. The solution resistance Rs remains almost stable at around 14 U during cycling. The diffusion resistance Zw also decreases from 427 U to 160 U upon cycling favouring enhanced cyclability. The exchange current density I0 increases from 0.06 mA cm2 to 0.12 mA cm2 after 200 cycles favouring electrochemical cycling. Fig. 7d shows the first cycle charge/discharge voltage profile at a
Fig. 7. (a) Impendence spectra of PSC in SIB before and after cycling. (b) Rate capability studies of PSC at different current densities. (c) Cycling capability of PSC at current densities of 0.1 Ag-1 and 0.4 Ag-1. (d) Galvanostatic charge and discharge profile of PSC for the 1st, 2nd and 100th cycle at 0.1 Ag1.
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Fig. 8. Mechanism of Li/Na insertion in PSC.
current density 0.1 Ag-1. It displays similar electrochemical voltage behaviour to LIB. The presence of a plateau at approximately 0.74 V in the first cycle may be endorsed to the formation of SEI films on the electrode surface. The CV study has been provided in the Supporting document (Fig. S10 b). The charge specific capacity in the first cycle of the SIB is about 660 mAh g1 while the discharge capacity is 370 mAh g1 with 56% columbic efficiency (Fig. 7d). The low initial columbic efficiency is a well accepted phenomenon in carbon nanostructures which can be related to the irreversible capacity loss due to the formation of SEI and irretrievably trapped Na ions in the porous structure. The columbic efficiency increased to above 98% in the subsequent cycles. PSC electrode exhibits excellent rate capability when cycled at different current rates as in Fig. 7b. The first discharge specific capacities at 0.1, 0.2, 0.6, 0.8, 1.6, 2 and 0.4 A g1 current density are 370, 283,242,199,127,107 and 247 mAh g1 respectively. PSC also exhibits excellent cycle life with a high capacity of ~325 mAh g1 at a current density of 0.1 A g1 for 200 cycles with a columbic efficiency of 99.9% (Fig. 7c). At 0.4 A g1 current density, the reversible capacity of the electrode material is 234 mAh g1 for 150 cycles. It should be pointed out that the cycling stability as well as capacity achieved using PSC is superior to other similar works reported in literature [17,21]. Comparisons of the present work and recent similar works on biomass derived material is provided in the supporting data (Tables S2 and S3). The TEM images provided in the supplementary data (Fig. S3) of PSC after cycling shows that the porous structure is retained with little damage even after 200 cycles implying excellent structural stability of the material. The hierarchical porous structure of the PSC and the charge/ discharge pathway for Li/Na ions are schematically illustrated in Fig. 8. PSC has highly complex hierarchical arrangement of macromeso-micro pores, linked with short pathways for rapid ion diffusion and large number of active sites for Li/Na ion accommodation. While some ions can intercalate between the layers, others can be inserted into the active pores. Absorption of ions onto the surface of meso pores is also possible which not only provide higher capacity, but also rapid energy storage. Macro pores can help in easy flow of electrolyte into the structure, thereby minimizing the ion diffusion distances. In addition, surface defects induced by N-doping and associated O atoms in carbon also enhances conductivity and battery performance. The unique porous structure can also accommodate the local volume change of the material, effectively reducing the pulverization of anode and improving cycle life.
4. Conclusions In this study, N-doped hierarchically micro-meso-micro porous carbon has been synthesized from low cost, environmental friendly, natural waste material viz. shells of Indian prawn and demonstrated as a potential anode material for both Li and Na ion batteries. PSC delivers a steady state stable specific capacity of 732 mAh g1 at 0.1 Ag-1 (0.26C rate) and 410 mAh g1 at 2C for 150 cycles when used as anode for LIBs. Excellent rate capability is also exhibited by the material even when tested at very high current rates. PSC also shows excellent electrochemical performance in SIBs with a very stable specific capacity of 325 mAh g1 at 0.1 Ag1 for 200 cycles and 234 mAh g1 at 0.4 Ag-1 for 150 cycles. This superior electrochemical performance of PSC anode over other biomass derived carbons can be attributed to the presence of hierarchical porous structure and N-doping of carbon. We believe that this work on prawn shell derived N-doped carbon (PSC) is absolutely unprecedented and entirely worthy material for making a revolution in energy storage field. Acknowledgements One of the authors (Indu Elizabeth) acknowledges CII, Eon Electric Ltd. and SERB for funding her research work under Prime Minister's Fellowship for Doctoral Research. The authors are also thankful to CSIR for funding the research under the CSIR-TAPSUN program. Thanks to Dr.Manjusha Shelke, Scientist, CSIR-NCL,Pune for carrying out the BET analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.08.106. References [1] D. Linden, T.B. Reddy, in, McGraw-Hill Companies, Inc., New York2002,(1.3e1.17, 7.10-7.11, 22.12-22.13), 1865. [2] P. Ge, M. Fouletier, Solid State Ion. 28 (1988) 1172e1175. [3] I. Elizabeth, R. Mathur, P. Maheshwari, B. Singh, S. Gopukumar, Electrochim. Acta 176 (2015) 735e742. [4] F.D. Han, Y.J. Bai, R. Liu, B. Yao, Y.X. Qi, N. Lun, J.X. Zhang, Adv. Energy Mater. 1 (2011) 798e801. [5] Y. Yan, Y.X. Yin, Y.G. Guo, L.J. Wan, Adv. Energy Mater. 4 (2014) 1301584e1301589. [6] D. Larcher, J. Tarascon, Nat. Chem. 7 (2014) 19e25. [7] X. Meng, P.E. Savage, D. Deng, Environ. Sci. Technol. 49 (2015) 12543e12550. [8] L. Zhang, Z. Liu, G. Cui, L. Chen, Prog. Polym. Sci. 43 (2015) 136e164. [9] G. Xu, J. Han, B. Ding, P. Nie, J. Pan, H. Dou, H. Li, X. Zhang, Green Chem. 17
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Bio-derived hierarchically macro-meso-micro porous carbon anode for lithium/sodium ion batteries Indu Elizabeth a, b, c, **, Bhanu Pratap Singh b, c, d, Sunil Trikha e, Sukumaran Gopukumar a, c, d, * a
CSIR-Central Electrochemical Research Institute, Karaikudi, India Physics and Engineering of Carbon, CSIR-National Physical Laboratory, New Delhi, India Academy of Scientific and Innovative Research, India d CSIR- Network Institutes for Solar Energy, India e EON Electric Ltd, Noida, India b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Prawn shells are converted into porous carbon through economically viable process. The derived carbon exhibits excellent capacity in both Li/Na ion batteries. NMR (Li6, Li7 & Na 23) and EPR studies on intercalated derived carbon carried out.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 May 2016 Received in revised form 22 August 2016 Accepted 25 August 2016
Nitrogen doped hierarchically porous carbon derived from prawn shells have been efficiently synthesized through a simple, economically viable and environmentally benign approach. The prawn shell derived carbon (PSC) has high inherent nitrogen content (5.3%) and possesses a unique porous structure with the co-existence of macro, meso and micropores which can afford facile storage and transport channels for both Li and Na ions. PSC is well characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Transmission electron Microscopy (TEM), High resolution TEM (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Electron Paramagnetic Resonance (EPR) and Solid state-Nuclear Magnetic Resonance (NMR) studies have been conducted on pristine PSC and Li/Na interacted PSC. PSC as anode for Lithium ion batteries (LIBs) delivers superior electrochemical reversible specific capacity (740 mAh g1 at 0.1 Ag-1 current density for 150 cycles) and high rate capability. When used as anode material for Sodium ion batteries (SIBs), PSC exhibits excellent reversible specific capacity of 325 mAh g1 at 0.1 Ag-1 for 200 cycles and rate capability of 107 mAh g1 at 2 Ag-1. Furthermore, this study demonstrates the employment of natural waste material as a potential anode for both LIB and SIB, which will definitively make a strike in the energy storage field. © 2016 Published by Elsevier B.V.
Keywords: Lithium ion batteries Sodium ion batteries Anode Bio carbon Specific capacity NMR
* Corresponding author. CSIR-Central Electrochemical Research Institute, Karaikudi, India. ** Corresponding author. CSIR-Central Electrochemical Research Institute, Karaikudi, India, CSIR-National Physical Laboratory, New Delhi, India E-mail addresses: [email protected] (I. Elizabeth), gopukumar@cecri. res.in (S. Gopukumar). http://dx.doi.org/10.1016/j.jpowsour.2016.08.106 0378-7753/© 2016 Published by Elsevier B.V.
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1. Introduction Lithium ion batteries (LIBs) have become the universal power source for most of the portable consumer electronics since they were launched in 1981. Currently, they are the technology of choice to develop electric vehicles and large scale storage of renewable energy because of their high energy density and high voltage. But these applications call for higher specific capacities and faster rate performance. A typical modern LIB cell consists of a cathode made from a lithium-intercalated layered oxide, a graphite anode and an organic electrolyte. The battery operates following a “rocking chair” concept in which the Liþ ions shuttle between the anode and cathode through the electrolyte during the charge/discharge cycles [1]. In spite of extensive research on electrode materials for LIB, still they are lacking in terms of rate capabilities, capacities and economic viability. Novel highly structured and cheap electrode materials have to be investigated for improved performances. In this context, if LIBs are to gain a significant place in the future, there is going to be an increasing demand for Li, resulting in concerns about the increasing cost and limited abundance of Li. Sodium ion battery (SIB) is one possible alternative to LIB owing to its low cost and higher natural abundance of sodium particularly when large scale applications are considered. The state of the art anode for LIB is graphite but exhibiting low capacities when used in SIBs as only a small amount of Na can intercalate into graphite [2]. Disordered carbon with porous morphology appears to be very promising as anode for SIBs. Many carbonaceous materials with various morphologies and structures like carbon nanotubes (CNTs) [3], hollow nanospheres [4], graphene [5] etc. have been investigated as anode materials for rechargeable batteries. However, the complex synthesis methods, expensive equipments and the use of toxic/harmful chemicals make it less attractive. Recently, carbonaceous material derived from biomass has been explored widely as anode material for LIBs [6e9]. Biomass derived carbon has the advantage of being a renewable, low cost source, and also can provide excellent porous 3D nanostructure which is of very particular interest for high lithiation/sodiation capacity and excellent cyclability in batteries [10]. N-doping of carbon is proven to improve the electrochemical performance, as it enhances the electrical conductivity and also provide more sites to increase the interaction between carbon and Li/Na during cycling. N-doped carbons are synthesized typically using various expensive, toxic N containing precursors following rigorous/complex experimental procedures. In this point, preparation of N-doped carbon from naturally occurring bio waste/ biomass is very appealing as it reduces the environmental pollution, lessens the usage of toxic chemicals and is very inexpensive both in terms of source cost and processing cost. Several studies have been reported recently on biomass derived anode material for LIB/SIBs like carbon material derived from human hair [11e13], ox horn [14], agricultural waste [15], peanut shells [16,17], wheat straw [18], banana peels [19], coconut shell [20], fish scales [21] and plant biomass [8,22,23]. In this work, we have synthesized a hierarchical macro-mesomicro porous, turbostractic, N-doped carbon material derived from marine Indian prawn shells. The Indian prawn (Fenneropenaeus indicus, formerly Penaeus indicus) [24], is one of the major commercial prawn species of the world. F. indicus is known by many common names around the world, including Indian white prawn, Tugela prawn, white prawn, banana prawn, Indian banana prawn and red leg banana prawn. About 10 10 to 10 11 tons of prawn shells are produced each year as natural waste. In addition, capture fisheries and aquaculture also contributes enormous quantity of prawn shells as waste side product [25,26]. Prawn/shrimp
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crustacean shells contains nitrogen containing polysaccharide, chitin (poly-b(1-4)-N-actetyl eD-glucosamine) and inorganic components mainly CaCO3. Hence, prawn shells are an inexpensive, easily available, abundant source of nitrogen containing biomass which can be put into use for a wide variety of applications. N edoped highly porous carbon has been synthesized from prawn shell by a very simple method, which is not only economical but also environmental friendly. Porous carbon derived shrimp/ prawn shells has been used for applications like supercapacitors [27,28] and catalyst for oxygen reduction reaction (ORR) [29] recently. We have evaluated carbon obtained from prawn shells as anode material for LIBs and SIBs and obtained excellent electrochemical performances. Herein, we have successfully converted a waste material to a wonder/wealth material for high end applications. To the best of our knowledge, this is the first time prawn shell derived carbon is used as electrode materials in both LIB and SIBs, which we believe will revolutionize the present energy scenario. 2. Experimental section All the chemicals used in the study were of analytical grade from Sigma-Aldrich and Merck and used as received without further purification. De-ionized (DI) water was prepared on site using Millipore-Q water system. The Indian prawn shells used in this study were collected from the local fish market (Karaikudi, India), a waste material. 2.1. Preparation of prawn shell derived porous carbon (PSC) The Indian prawn shells were washed thoroughly with water and dried in air. The material was heated at 300 C and grinded. The pre-carbonized material was immersed in 0.1 M NaOH solution for 24 h followed by heating at 750 C under inert argon atmosphere for 2 h with a heating ramp of 3 C min1. NaOH is used as an activation agent for creating porous structure by leaching out the acidic impurities. The obtained black material was washed extensively with 1 M HCl to remove the metal impurities like Ca, followed by washing with DI water till neutral pH was attained. The resulting carbon material was dried at 80 C under vacuum. The asfabricated prawn shell derived carbon material is designated as PSC hereafter. 2.2. Material characterization The surface morphology of the synthesized powder was examined using a FESEM (Model: KARL ZEISS) and HRTEM (Tecnai 20 G2(FEI). XPS of the synthesized powder has been performed using a MULTILAB 2000 (Thermo Scientific) photoelectron spectrometer and the spectra were recorded using an X-ray source (MgKa radiation 0e1253 eV). XRD have been carried out with a Bruker D-8 diffractometer equipped with a Cu Ka radiation (1.5406 Å) operated at 40 kV and 30 mA. Raman spectroscopy has been carried out at room temperature with Green LASER (excitation line 514 nm at 2.5 mW power) using Renishaw InVia Reflex Micro Raman spectrometer equipped with a CCD detector. EPR studies of PSC before and after interaction with Li and Na, was conducted using EPR spectrometer (Model: BRUKER EMX Plus) at microwave frequency of 9.863,940 GHz and 10 mW power at room temperature. 6Li, 7Li and 23Na NMR of PSC after intercalating with Li/Na was recorded using NMR instrument (BRUKER, 400 MHz). N2 sorption analysis was performed at 77.4 K using QUANTASORB equipped with surface area and pore size analyzer. Sample was degassed at 150 C for 24 h before analysis. BET surface area was determined according to BET model. The electrochemical measurements were carried out by
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fabricating 2032 coin type cells in an argon-filled glove box with Li/ Na metal as counter as well as reference electrodes, for LIB and SIB respectively. The working electrode slurry was prepared by using 80% PSC, 10% super-P carbon, and 10% PVDF (polyvinylidenefluoride) binder blended with NMP (N-methyl-2-pyrrolidone) solvent and coated over a Cu foil. The polypropylene separator was soaked in an electrolyte (LiPF6 in 1:1 EC: DEC for LIB and in house synthesized NaPF6 in EC/DEC for SIB) and sandwiched between the electrodes. The galvanostatic charge/discharge studies were carried out at different C-rates with the assembled coin cells using an automatic battery cycle life tester (WONATECH, Korea) between 0.01 and 3 V. Electrochemical impedance studies (EIS) were carried out using Frequency Response Analyzer (Biologic, USA) with an AC signal of 5 mV in the frequency range from 100 kHz to 5 mHz. Post-mortem of the cycled cells was done to study the structure intactness of the anode material. The anode samples were taken out from the cell and cleaned using the electrolyte solution inside the glove box and TEM studies were conducted. 3. Results and discussions The (002) region of XRD spectrum of PSC (Fig. 1a) exhibits a
superposition of a weak and broad amorphous carbon peak, particularly towards the lower angle range (around 2q ¼ 23 ) and a strong graphitic peak at 26.6 . Inset of Fig. 1a shows two distinct deconvoluted peaks. In addition, a weak (100) band is also observed at ~ 40 which also points to the presence of crystalline carbon or graphite like structure in PSC. These observations suggest that carbon derived from prawn shell has an intermediate structure between graphite and amorphous called turbostratic structure, in which the graphene layers randomly translate to each other [30,31]. The d002 spacing calculated using the Bragg equation from the angular location of the strong peak is 0.35 nm which matches with turbostratic structure [32,33]. Fig. 1b depicts the Raman spectrum for the synthesized PSC. Two peaks are noted at 1347 cm1 (D band) and 1594 cm1 (G band) corresponding to the structural defects and disorder induced in the graphene layers of carbon and the first order stretching vibration of sp2 carbon atoms in the two dimensional hexagonal lattice respectively [34]. The intensity ratio of D-band to G-band (ID/ IG) is 1.02 which indicates the presence of defects and imperfections in the structure. Partial replacement of carbon atom by nitrogen in PSC can also promote defects. A broad 2D is also seen in the Raman spectra at ~2800 cm1 implying layering of graphene
Fig. 1. (a) XRD pattern and (b) Raman spectra of PSC and (c) Nitrogen adsorption-desorption isotherm of PSC. Inset - the pore size distribution of PSC (magnified image provided in supporting document Fig. S5).
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sheets (turbostractic structure) [35]. N2 adsorption-desorption isotherm at 77.4 K (Fig. 1c) presents typical type IV shape with H2 hysteresis located at a relative pressure of 0.4e0.99, which is characteristic for meso-porous structure [36]. The specific surface area of PSC is 336 m2 g1. The comparatively lower BET surface area of PSC can reduce the unwanted irreversible side reactions and lessen the irreversible capacity loss [37]. The pore size distribution of PSC [inset of Fig. 1c] shows mainly meso-pores in the size range of 2e10 nm and also some meso pores with ~20 nm diameter. It is clearly visible that PSC is also composed of micro-pores of size <2 nm. Also, some macro pores of size around 50 nm are observed. The hierarchically porous nature of PSC is more clearly confirmed from the TEM and HRTEM studies. The uniqueness of PSC nanostructure was studied using SEM, FESEM, TEM and HRTEM. Fig. 2a depicts the SEM image of PSC from a macroscopic perspective, showing PSC consisting of nearly spherical particles of diameter around 1 mm. However, from the images taken under higher magnification, FESEM (Fig. 2b) and TEM (Fig. 2c and d) exhibit more complex carbon structure. Fig. 2b represents that the PSC particles are made up of graphene analogue carbon sheets which are randomly stacked. Highly porous graphene analogue sheets of size ~500 nm randomly arranged with some degree of graphitization are clearly visible in TEM images (Fig. 2c and d). Further examination under higher magnification, the PSC structure has been found to be more exciting. The TEM images (Fig. 3a, b and c) reveals that the graphene analogue carbon nano sheets are made up of clusters of meso porous, spherical carbon nano sheets (Meso-CNS) of size around 20 nm. Meso-CNS are
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arranged in such a way that there are macro pores between them which can act as ion buffering reservoirs and also help in better electrolyte penetration [38]. Interestingly, it is important to mention here that the presence of meso-pores in Meso-CNS offers the possibility of fast ion transfer by reducing the transportation path thereby favours the penetration of electrolyte to realize better performance of the electrode material [12] and provides ideal surfaces for the adsorption of Li/Na ions with a very small innerpore resistance [38e40]. A closer inspection of Meso-CNS using HRTEM (Fig. 3d, e and f) shows micro-pores and localized graphite layers, which are the typical for turbostratic carbon structure. The inset of Fig. 3e is the Selected Area Electron Diffraction (SAED) pattern which clearly shows some crystallinity due to turbostratic layering of carbon as confirmed from XRD. It is highly plausible that such a structure will provide additional intercalation sites for accommodation of Li/Na ions, resulting in the higher specific capacity. PSC with its unique, magnificent, hierarchical macro-meso-micro porous structure is expected to exhibit excellent electrochemical performance in both SIBs and LIBs. X-ray photoelectron spectroscopy (XPS) has been used to quantify the elemental composition of PSC. As depicted in Fig. 4 the XPS survey spectrum exhibits three major peaks at ~284, 400 and 532 eV corresponding to C1s, N1s and O1s respectively. The peak assignment and relative atomic percentages are tabulated in Table S1 (Supporting data). High resolution XPS analysis of N1s photoelectron envelope indicated the presence of structurally integrated pyridine (398.35 eV; area 36.2%) and pyrrolic N (400.04 eV; area 63.8%) [14]. In the table, it can be seen that the mass percentage of doped nitrogen is as high as 5.3%. The
Fig. 2. (a) SEM image of Prawn shell derived carbon (PSC) (b) FESEM image of PSC showing graphene analogue carbon sheets stacked (turbostratic structure) (c) and (d) TEM image of PSC showing porous graphene analogue sheets.
Fig. 3. (a), (b) and (c) TEM images of PSC. (d), (e) and (f) HRTEM images of PSC.
Fig. 4. (a) XPS survey spectrum (b) C1s, (c) N 1s and (d) O 1s spectra of PSC.
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electrochemical performance of PSC material is improved by the high N content as it enhances the electronic conductivity as well as formation of defects which can act as active Li/Na storage sites [14]. As shown in XPS, C1s spectrum (Fig. 4b) the most characteristic peak of carbon (sp2 C]C) is located at 284.8 eV and the remaining peaks are sp3 C (284.29 eV), CeN nitrile (285.50 eV), pyrolic carbon linked to nitrogen CeN/CeO (286.40 eV) and C]O (288.29 eV). The total C content in PSC is around 78%.The fractional percentage of various groups in C 1s spectra (Table S1) indicates that the PSC has large number of structural defects that can support rapid charge etransfer reactions and act as Li/Na insertion sites, thereby improving the electrochemical performance. The O1s spectrum indicates the presence of three types of oxygen bonds eC]O (530.73 eV), OeCeO (532.06 eV) and OeC]O (533.33 eV) [14]. The oxygen mainly comes from the thermally stable groups in carbon except for the absorbed oxygen or moisture on the carbon surface. The oxygen as doped heteroatom can play a positive role for Li storage by increasing defects, disorder, or local electron density around O atoms according to previous studies [41]. EPR technique is a useful tool for understanding the mechanism of reaction of alkali metals with carbon as it can detect the localized paramagnetic centres assigned to p aromatic radicals, which can react electrochemically with Li and Na [42e44]. The EPR spectra of pristine PSC are presented in Fig. 5. An isotropic signal with Lorentzian curve shape with g ¼ 2.0022 and line width of ~0.36 mT, which originates from localized paramagnetic centres is observed. The origin of these localized radicals can be attributed to the imperfect carbon structure, such as dangling bonds with unpaired electrons on the edges of small esize porous graphene sheets (as seen in TEM) [45].
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After electrochemical intercalation with Li and Na (Fig. 5b and c), the EPR signal coming from the original localized radicals are perturbed. Similar feature has been observed earlier in the case of petroleum cokes treated below 1000 C [43] and in surface modified Carbon Microbeads [46], wherein the reason is ascribed to the reaction of Li/Na with dangling bonds and hydrogen in the edge of graphene layers, resulting in the formation of covalent bonds, contributing to the disappearance of this signal [45]. Electrochemical reaction of PSC with Li/Na results (Fig. 5b and c) in broader EPR signals with one order of magnitude higher than that of original localized radicals with nearly same g value (about 2.0028). The signal can be considered as the combined contribution from localized paramagnetic centres with different local environments. For instance, localized paramagnetic centres can be formed due to Li/Na interaction with the edges of graphene layers or Li/Na intercalation between the turbostratic layers or Li/Na adsorption on the surface of small-size disordered graphene sheets [46]. 6Li and 7Li NMR studies of lithiated PSC (Supporting document Fig. S1) also shows the evidence of different environments of inserted Li. 23 Na NMR studies given in the supporting document (Fig. S2) also show the PSC is suitable for Na ion intercalation also. Comparatively lesser intensity of EPR signals from Na interacted PSC can be attributed to the lower amount of Na that can be accommodated in the PSC structure. The Dysonian asymmetry in the line shape (Table 1), characterized by high ratio of A/B, low field peak to high field peak amplitudes is observed in Li/Na interacted PSC [47]. This indicates an increased number of conduction electrons in the sample after Li/Na intercalation [48].
Fig. 5. EPR spectra at room temperature of (a) PSC (b) Li intercalated PSC and (c) Na intercalated PSC. (d) g-value of PSC and Li/Na intercalated PSC.
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Table 1 EPR parameters of PSC, Li interacted PSC and Na interacted PSC. Sample
Signal intensity I (a.u.)
EPR line width DHpp (mT)
Dysonian asymmetry (A/B)
PSC PSC/Li PSC/Na
4.8 162 91
0.36 1.34 1.25
~0.13 ~1.04 ~1.02
3.1. Electrochemical studies 2032 type coin cells were assembled to evaluate the electrochemical performance of PSC in LIBs. In order to understand the interfacial reaction between the electrode and electrolyte and investigate the transport kinetics involved in electrochemical performance Electrochemical Impedance Studies (EIS) measurements were conducted for the Li ion coin cell before and after cycling in the frequency range 100 kHz to 5 mHz. As depicted in Fig. 6a, the Nyquist plots of PSC consist of semicircle in the high and middle frequency regions and a straight line in the low frequency region. The matching equivalent Randles circuit has also been provided as inset in Fig. 6a. Here Rs is the solution resistance of the cell which includes resistance from the electrolyte; Cdl is the double layer capacitance; Rct is the charge transfer resistance; and Zw is the Warburg impedance related to the lithium diffusion in the electrode [49,50]. EIS spectra is fitted and the corresponding parameters were derived which is summed up in Table 2. The solution resistance Rs does not show any variation indicating the electrical stability of the electrolyte. The charge transfer resistance, Rct, decreases after cycling which indicates an
improvement of interfacial conductivity for fast charge transfer between the electrode and electrolyte. The decrease is because of the large irreversible Li insertion into the electrode during cycling owing to its surface area and porosity thereby improving the conductivity [51]. The value of double layer capacitance (Cdl) remains almost constant during the cycling implying formation of a stable SEI. The exchange current density (I0) can be calculated according to Equation (1).
I0 ¼ RT=nFRct
(1)
Where R is the gas constant, T is the absolute temperature, n is the number of transferred electrons and F is the Faraday constant. I0 is an important parameter influencing the rate of a charge transfer controlled electrochemical reaction [52]. Here, the increase in I0 with cycling indicates a favourable charge transfer process of Li ion insertion/extraction reaction. There is also decrease in the diffusion resistance, Zw, after cycling demonstrating that the structure of PSC is suitable for Li ion diffusion leading to superior electrochemical performance. The galvanostatic charge/discharge performance of PSC has
Fig. 6. (a) Impendence spectra of PSC before and after cycling. In the inset is equivalent Randles circuit. (b) Rate capability studies of PSC at different current densities. (c) Cycling capability of PSC at current densities of 0.1 Ag-1 and 0.75 Ag-1. (d) Galvanostatic charge and discharge profile of PSC for the 1st,10th and 100th cycle at 0.1 Ag1 and 1st cycle at 0.6 and 2 Ag-1 current densities in LIB.
I. Elizabeth et al. / Journal of Power Sources 329 (2016) 412e421 Table 2 EIS spectra fitting parameters of PSC anode in LIB. S.No
Parameters
Before cycling
After cycling
1 3 4 6 7
Rs (U) Rct(U) Zw (U) Cdl (F) I0 (mA cm2)
4.73 ± 0.274 145.1 ± 0.608 88.39 ± 0.301 3.56e-6±32e-9 0.177
4.70 ± 0.251 83.50 ± 0.918 55.03 ± 0.462 3.29e-6±44.4e-9 0.3079
been carried out at different current densities in the voltage range 0.01 Ve3 V such as shown in Fig. 6b for determining the rate tolerance of the material. When cycled at 0.1 Ag-1 current density the PSC electrode shows a high initial capacity of 1013 mAh g1, which is more than thrice the capacity of pure carbon signifying that there are other lithium storage ways in PSC other than conventional graphite intercalation. Although large irreversible capacity loss is observed in first two cycles, the capacity stabilizes after the second cycle to 740 mAh g1. The heavy capacity fade during the initial cycles can be attributed to the large amount of Li which is getting irreversibly trapped in pores of PSC and in the formation of SEI layer. In other words during the formation cycles (initial few cycles), the carbon electrode gets converted from its pristine state to an active Li storage host. The PSC electrode exhibits a reversible specific capacity of 740, 653, 436, 298 and 147 mAh g1 at 0.1, 0.2,0.8, 1.6 and 2 A g1 (5C) respectively (Fig. 6b). 1C rate is taken as 372 mA g1. After cycling at very high rate, the reversibility of the material was checked by cycling at current densities 1.2, 0.6 and 0.1 Ag-1. The initial reversible capacity (723 mAh g1) at 0.1 A g1 was recovered implying excellent rate capability of the material. Fig. 6d depicts the voltage
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vs. specific capacity profile of the electrode material at different rates. During the first charge cycle, at 0.1 A g1, plateau is observed around ~0.8 V corresponding to the SEI formation which is typical for carbonaceous material [3]. The appearance of low voltage plateau is mainly due to the filling of the pores by Li [53]. The appearance of plateau in the discharge profile around 0.25 V corresponds to the extraction of Li from PSC in the EC þ DEC electrolyte [54]. At high C rates the disappearance of the plateaus may be due to the high activation polarization developed while applying high current between the electrodes [51]. The life time performance of the PSC was tested by cycling the cell for 150 cycles at two different current rates namely 100 (0.25 C) and 750 (2 C) mA g1 as depicted in Fig. 6c. The electrode material shows a very stable specific capacity of 730 mAh g1 when cycled at 0.25 C with a coulombic efficiency of ~99.9% after the initial few cycles. At 2C rate, the PSC exhibits a specific capacity around 470 mAh g1, with excellent cycling stability up to 150 cycles, which is more than thrice the practical capacity of graphite. Owing to the developed meso/micro porosity and good electrical conductivity of PSC, it was evaluated as SIB anode by galvanostatic charge-discharge test in the voltage range 0.01 Ve3 V (Fig. 7). EIS spectra (Fig. 7a) shows a decrease in Rct resistance, from 420 U before cycling to 200 U after cycling for the investigated 200 cycles implying an increase in interfacial conductivity upon cycling. The solution resistance Rs remains almost stable at around 14 U during cycling. The diffusion resistance Zw also decreases from 427 U to 160 U upon cycling favouring enhanced cyclability. The exchange current density I0 increases from 0.06 mA cm2 to 0.12 mA cm2 after 200 cycles favouring electrochemical cycling. Fig. 7d shows the first cycle charge/discharge voltage profile at a
Fig. 7. (a) Impendence spectra of PSC in SIB before and after cycling. (b) Rate capability studies of PSC at different current densities. (c) Cycling capability of PSC at current densities of 0.1 Ag-1 and 0.4 Ag-1. (d) Galvanostatic charge and discharge profile of PSC for the 1st, 2nd and 100th cycle at 0.1 Ag1.
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Fig. 8. Mechanism of Li/Na insertion in PSC.
current density 0.1 Ag-1. It displays similar electrochemical voltage behaviour to LIB. The presence of a plateau at approximately 0.74 V in the first cycle may be endorsed to the formation of SEI films on the electrode surface. The CV study has been provided in the Supporting document (Fig. S10 b). The charge specific capacity in the first cycle of the SIB is about 660 mAh g1 while the discharge capacity is 370 mAh g1 with 56% columbic efficiency (Fig. 7d). The low initial columbic efficiency is a well accepted phenomenon in carbon nanostructures which can be related to the irreversible capacity loss due to the formation of SEI and irretrievably trapped Na ions in the porous structure. The columbic efficiency increased to above 98% in the subsequent cycles. PSC electrode exhibits excellent rate capability when cycled at different current rates as in Fig. 7b. The first discharge specific capacities at 0.1, 0.2, 0.6, 0.8, 1.6, 2 and 0.4 A g1 current density are 370, 283,242,199,127,107 and 247 mAh g1 respectively. PSC also exhibits excellent cycle life with a high capacity of ~325 mAh g1 at a current density of 0.1 A g1 for 200 cycles with a columbic efficiency of 99.9% (Fig. 7c). At 0.4 A g1 current density, the reversible capacity of the electrode material is 234 mAh g1 for 150 cycles. It should be pointed out that the cycling stability as well as capacity achieved using PSC is superior to other similar works reported in literature [17,21]. Comparisons of the present work and recent similar works on biomass derived material is provided in the supporting data (Tables S2 and S3). The TEM images provided in the supplementary data (Fig. S3) of PSC after cycling shows that the porous structure is retained with little damage even after 200 cycles implying excellent structural stability of the material. The hierarchical porous structure of the PSC and the charge/ discharge pathway for Li/Na ions are schematically illustrated in Fig. 8. PSC has highly complex hierarchical arrangement of macromeso-micro pores, linked with short pathways for rapid ion diffusion and large number of active sites for Li/Na ion accommodation. While some ions can intercalate between the layers, others can be inserted into the active pores. Absorption of ions onto the surface of meso pores is also possible which not only provide higher capacity, but also rapid energy storage. Macro pores can help in easy flow of electrolyte into the structure, thereby minimizing the ion diffusion distances. In addition, surface defects induced by N-doping and associated O atoms in carbon also enhances conductivity and battery performance. The unique porous structure can also accommodate the local volume change of the material, effectively reducing the pulverization of anode and improving cycle life.
4. Conclusions In this study, N-doped hierarchically micro-meso-micro porous carbon has been synthesized from low cost, environmental friendly, natural waste material viz. shells of Indian prawn and demonstrated as a potential anode material for both Li and Na ion batteries. PSC delivers a steady state stable specific capacity of 732 mAh g1 at 0.1 Ag-1 (0.26C rate) and 410 mAh g1 at 2C for 150 cycles when used as anode for LIBs. Excellent rate capability is also exhibited by the material even when tested at very high current rates. PSC also shows excellent electrochemical performance in SIBs with a very stable specific capacity of 325 mAh g1 at 0.1 Ag1 for 200 cycles and 234 mAh g1 at 0.4 Ag-1 for 150 cycles. This superior electrochemical performance of PSC anode over other biomass derived carbons can be attributed to the presence of hierarchical porous structure and N-doping of carbon. We believe that this work on prawn shell derived N-doped carbon (PSC) is absolutely unprecedented and entirely worthy material for making a revolution in energy storage field. Acknowledgements One of the authors (Indu Elizabeth) acknowledges CII, Eon Electric Ltd. and SERB for funding her research work under Prime Minister's Fellowship for Doctoral Research. The authors are also thankful to CSIR for funding the research under the CSIR-TAPSUN program. Thanks to Dr.Manjusha Shelke, Scientist, CSIR-NCL,Pune for carrying out the BET analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.08.106. References [1] D. Linden, T.B. Reddy, in, McGraw-Hill Companies, Inc., New York2002,(1.3e1.17, 7.10-7.11, 22.12-22.13), 1865. [2] P. Ge, M. Fouletier, Solid State Ion. 28 (1988) 1172e1175. [3] I. Elizabeth, R. Mathur, P. Maheshwari, B. Singh, S. Gopukumar, Electrochim. Acta 176 (2015) 735e742. [4] F.D. Han, Y.J. Bai, R. Liu, B. Yao, Y.X. Qi, N. Lun, J.X. Zhang, Adv. Energy Mater. 1 (2011) 798e801. [5] Y. Yan, Y.X. Yin, Y.G. Guo, L.J. Wan, Adv. Energy Mater. 4 (2014) 1301584e1301589. [6] D. Larcher, J. Tarascon, Nat. Chem. 7 (2014) 19e25. [7] X. Meng, P.E. Savage, D. Deng, Environ. Sci. Technol. 49 (2015) 12543e12550. [8] L. Zhang, Z. Liu, G. Cui, L. Chen, Prog. Polym. Sci. 43 (2015) 136e164. [9] G. Xu, J. Han, B. Ding, P. Nie, J. Pan, H. Dou, H. Li, X. Zhang, Green Chem. 17
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