Poly(carbazole) Coated Selenium@Conical Carbon Nanofibers

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Poly(carbazole) Coated Selenium@Conical Carbon Nanofibers Hybrid for Lithium-Selenium Batteries with Enhanced Lifespan Radha Mukkabla, _ Kuldeep, and Melepurath Deepa ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01382 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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ACS Applied Energy Materials

Poly(carbazole) Coated Selenium@Conical Carbon Nanofibers Hybrid for Lithium-Selenium Batteries with Enhanced Lifespan Radha Mukkabla,a Kuldeep,a Melepurath Deepa,a,* a

Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy502285, Telangana (India)

Keywords: Poly(carbazole); batteries; Selenium; capacity; polyselenides ABSTRACT A scalable protocol enabling Selenium (Se) cathodes in Lithium (Li)-Se batteries to retain a high reversible capacity with repeated cycling is demonstrated. A hybrid of Se powder with conical carbon nanofibers (CCNFs) labeled as Se@CCNFs-20, is prepared in a non-inert atmosphere at room temperature and is used as cathode in a Li-Se cell. Reversible capacity of ~990 mAh gSe-1 is achieved at 0.1 current (C)-rate for the Li-Se@CCNFs-20 cell, which reduces to ~531 mAh gSe-1 after 100 cycles. CCNFs are composed of elongated fibers with a graphitic crystalline structure; they maximize Se uptake by the virtue of their effective surface area, promote electron conduction between the Se particles by serving as conductive interconnects and accommodate the volume expansion of Se during discharge, thus manifesting in the above described

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performance. This performance is bettered by coating the Se@CCNFs-20 hybrid cathode with a conducting polymer (poly(carbazole) or PCZ) layer. The PCZ coating acts a barrier that not only restricts the dissolution and cross-over of polyselenides thus improving the capacity retention of the cell, but it also amplifies the rate performance by providing interfacial properties conducive for fast Li-ion reaction with the active Se content at the cathode. This results in a significant enhancement in the electrochemical charge storage properties of the Li-Se@CCNFs-20-PCZ cell compared to the Se@CCNFs-20 based cell. The PCZ coating minimizes the capacity fade, for the cell experiences a very slow capacity decay rate of 0.0051% per cycle from 10th cycle, finally preserving a reversible capacity of ~640 mAh gSe-1 at the end of 100 cycles at 0.1 C-rate, which is the highest reversible capacity ever reported for a Li-Se cell cycled under the said conditions. High Se utilization, low polarization, and durability with an ultra-high Se loading are imparted to the Se cathode by the PCZ overlayer, thus opening up the possibilities for scale-up for practical applications. INTRODUCTION Rechargeable battery alternatives based on new materials are sought in view of the extraordinarily high capacities they can offer in comparison to the less capacitive but stable and long lived lithium-ion batteries (LIBs). Besides portable electronic devices, for batteries to be incorporated in advanced applications such as electric vehicles (EVs), next generation hybrid EVs, aviation machinery and so forth, they must possess capacity and gravimetric energy density much greater than what LIBS can offer. Commercial batteries typically employ LiFePO4 or LiCoO2 or LiMn2O4 based cathodes,1-3 and their usable energy densities are less than 200 Wh kg1

, thus preventing their adoption into EVs or heavy equipment.4 New battery technologies such

as Li-sulfur (S),5 Li-selenium (Se)6 and Li-O2 capable of delivering theoretical capacities greater


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than 500 mAh g-1, and energy densities in excess of 1000 Wh kg-1, are now being investigated by researchers worldwide to facilitate their commercialization in near future. While the Li-Se battery is an analogue of the Li-S battery, but Se has a distinct advantage over S. The electronic conductivity of Se is approximately 25 orders of magnitude greater than that of sulfur (σSe: 1 × 10-5 and σS: 1 × 10-30 S cm-1),7 which allows faster kinetics, and therefore, the Li-Se battery can deliver high capacities even when it is charged at high current rates. Apart from having a theoretical energy density 3 times greater than Li-ion and a significantly higher capacity of 678 mAh g-1, the Li-Se battery is also safe, maintenance free, and permits the usage of all its’ stored energy till full discharge. Se is also not very costly, when it is procured in bulk quantities.8 However, akin to the issues faced by Li-S batteries, the Li-Se battery also experiences severe capacity fade with cycling, due to the formation of the higher order polyselenides during discharge, which migrate to the Li-anode during discharge via the electrolyte. The detachment of polyselenide anions from the cathode, and the volume expansion incurred by Se during the formation of Li2Sen (n ≈ 3-8) and Li2Se2 or Li2Se, causes swelling and pulverization of the Se electrode, thus resulting in fast capacity decay due to this uncontrolled active material loss. To offset the leaching out of active Se from the cathode so as to prolong the operational lifespan of the Li-Se battery, the Se cathode is usually modified by confining Se particles within the pores of carbon nanomaterials. Multiwalled carbon nanotubes (MWCNTs),9 graphene,10 reduced graphene oxide (RGO),11 porous carbon materials (CMK-3, mesoporous C, microporous C, porous carbon nanospheres (PCNs)),

12,13

and conducting polymers like poly(pyrrole) (PPy),

poly(aniline) (PANI)14 have been used in the past. The voids or pores in the carbon or polymer matrix accommodate the volume expansion during Li2Sen formation, the high surface area of nanostructured carbons allow high Se loading, and since carbon is electrically conductive, it also


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assists in improving material utilization across the cross-section of the cathode. Other novel approaches include trapping of polyselenides by the use of a metal oxide such as TiO2,15 insertion of carbon layer between the working electrode and separator16 and the use of nanofibrous-17 and nano-porous- Se.18 These methods have been fruitful to some extent in improving the capacity retention of Se cathode by restricting the redox shuttle and dissolution of the polyselenide anions. For instance, a Li-3D-hierarchical graphene-CNT@Se cell delivered a capacity of 504.3 mAh g-1, after 150 cycles at 0.2 C-rate.8 A Se/PCNs composite with a 70.5 wt% Se loading delivered an initial volumetric capacity of 3156 mAh cm-3, which underwent a 0.03% capacity fade per cycle, over 1200 cycles at 1 C-rate.12 In a previous study by our group, a Li-Se/graphitic nanoplatelet nanofiber composite based cell was prepared, and it showed an initial capacity of 847.6 mAh g-1, and a capacity of 489 mAh g-1 was retained after 200 cycles at 0.1 C-rate.19 However, at a high C-rate of 0.5, the same cell retained a capacity of only 159 mAh g-1 after 100 cycles. A Se-impregnated nano-cellulose derived monolithic carbon based Li-Se cell exhibited a reversible capacity of 1028 mAh cm-3 (620 mAh g-1) and 82% retention over 300 cycles.8 Considering the challenges faced by the Li-Se battery, here we present a rational design for a Se cathode that not only delivers high capacity and endures repetitive cycling by resisting compositional degradation, but is also easy to prepare and to scale-up for practical applications. By dry-grinding commercially available Se powder with conical carbon nanofibers (CCNFs), the Se@CCNFs-20 hybrid delivers an electrochemical energy storage performance improved in comparison to commercial Se. However, most strikingly, by using an oversimplified approach of applying a coating of a conducting polymer, namely, poly(carbazole) or PCZ over the Se@CCNFs hybrid cathode, a dramatic enhancement is achieved in terms of capacity, rate


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capability and cyclability for the corresponding Li-Se@CCNFs-20-PCZ cell. PCZ is prepared by an oxidative chemical polymerization method at room temperature with inexpensive reagents, and can be prepared in huge quantities with ease. It is electrically conducting in the assynthesized or doped state, and is completely miscible in organic solvents like ethanol without any particulate residues, which makes its’ deposition over the Se@CCNFs-20 cathode facile. Further, it not only serves as a barrier layer that limits the polyselenides’ cross-over and loss via dissolution in electrolyte during repeated charge-discharge cycles but it also acts an active interfacial layer that maximizes the reaction of Li-ions from the electrolyte with the Se content at the cathode. The hetero-atom, i.e., the nitrogens in the PCZ backbone possibly function as sites that have affinity for Li-ions, and vis-à-vis these interactions, the accessible Se sites in the cathode are utilized more efficiently when the PCZ layer is present in the cell. This report provides a detailed study on how the CCNFs and the PCZ layer work in tandem to hold the cathode constituents in place and in ameliorating the operational lifetime of the Li-Se battery. Experimental Chemicals Selenium powder, conical carbon nanofibers (CCNFs) (graphitized, carbon basis, D × L 100 nm × 20-200 µm, >98%) , acetylene black, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), lithium nitrate (LiNO3), lithium metal, sodium alginate, ferric chloride (FeCl3), carbazole monomer were purchased from Sigma Aldrich and used as such. Acetone, chloroform and ethanol were bought from Merck. Ultrapure water (resistivity ~ 18.2 MΩ cm) was obtained through a Millipore direct Q3UV system. GF/D glass microfiber filters and stainless steel (SS) foils were obtained from Alfa Aesar. Preparation of poly(carbazole) (PCZ)


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Carbazole monomer (1 g) was added to 40 mL of chloroform and the solution was magnetically stirred for 5 min. and a clear pale yellow colored solution was obtained. An aqueous solution of the oxidant, FeCl3 (2 g) in 3 mL of deionized water was introduced into the monomer solution, in a drop-wise manner, while the monomer solution was continuously being stirred. After the complete addition of FeCl3, the stirring was continued for 24 h at room temperature. A green colored precipitate was obtained, which was collected by filtration, washed with water and ethanol several times, and dried in a vacuum oven at 60 °C for 6 h. The final green colored dried powder is labeled as the poly(carbazole) (PCZ). Scheme 1 shows the preparation of PCZ. Fabrication of Se based cathodes and Li-Se cells Commercial selenium powder and CCNFs were mixed together in a weight ratio of 3:1, using a mortar and pestle, and dry-grinded for 2 h at room temperature. The resulting black powder is labeled as a Se@CCNFs-20 hybrid. Two more hybrids were prepared by using the same procedure but with different weight proportions of Se powder and CCNFs: 7:1 and 1.66:1. Commercial Se metal powder was also grinded for the same duration without CCNFs, and it is referred to as pristine Se or Se. The Li-Se battery electrodes are prepared by first mixing the active material (Se@CCNFs hybrid), acetylene black and sodium alginate in weight ratios of (70:10):10:10, (60:20):10:10 and (50:30):10:10. The weight ratios in parenthesis represent the Se:CCNFs ratios, which are 7:1, 3:1, and 1.66:1 respectively. A few drops of deionized water were added as the solvent to make a uniform homogeneous slurry. The components were grinded in a mortar and pestle for 30 min. The slurry was coated on circular stainless steel foils of 12 mm diameter with the help of an overhead projector (OHP) sheet. It is a transparent sheet made of plastic or high quality poly(vinyl chloride), which is used to apply the active material slurry on the current collector. The electrodes are labeled as Se@CCNFs-10, Se@CCNFs-20 and


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SE@CCNFs-30 respectively. For preparing the pristine Se electrode, Se powder, acetylene black and sodium alginate were mixed in an 80:10:10 weight ratio. For preparing the ternary cathode, PCZ (1 mg) was dissolved completely in ethanol (10 mL) and the ensuing clear green colored PCZ solution was carefully drop cast on the Se@CCNFs-20 hybrid electrode using a micro-pipette, such that 0.025 mg of PCZ was deposited over a given electrode. All the electrodes were dried at 60 °C in a vacuum oven for 12 h to remove the residual solvents. The cathode with a PCZ coating is referred to as the Se@CCNFs-20-PCZ electrode and this fabrication can also be visualized from Scheme 1. The loading density of the materials was maintained in the range of 1.5 to 2.0 mg cm-2. Swagelok cells are used for testing and assembled in the argon filled glove box (O2 ≤ 0.5ppm, H2O ≤ 0.5 ppm LAB star Mbraun) by using pristine Se or Se@CCNFs hybrids or PCZ coated hybrid as the working electrode, few drops of 1 M LITFSI and 0.2 M of LiNO3 (additive) dissolved in a mixture of DOL/DME (1:1 by volume) as the electrolyte wetted with GF/D borosilicate glass fiber as the separator, and lithium foil as counter/reference electrode. Instrumentation Galvanostatic charge-discharge curves and cycling stability of Li-Se cells were tested electrochemically with an Arbin BT2000 within the voltage range of 1.65 - 3 V versus Li+/Li at 0.1 C-rate. The rate capability test was performed at 0.1, 0.2, 0.3, 0.5, 1 and 2 C-rates on the same instrument. Cyclic voltammogram (CV) plots of the cells were recorded by using Autolab PGSTAT 302N potentiostat/galvanostat, equipped with a NOVA 1.9 software at a scan rate of 0.2 mV s-1 in the voltage range of 1.65 - 3 V versus Li+/Li. Linear sweep voltammograms (I-V) plots of the materials were recorded at 10 mV s-1, on the same instrument. All the electrochemical measurements were carried out at room temperature.


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The crystalline phase of the active materials were identified by X-ray diffraction by using a PANalytical X’pert PRO (Netherlands) machine with Cu-Kα (λ=1.5406 Å) radiation as the Xray source operated at by applying an accelerating voltage of 40 kV and 30 mA current. The morphological studies on the materials were performed on a scanning electron microscope (SEM Carl Zeiss EVO18). Transmission electron microscopy (TEM JEOL 2100 microscope) was performed on the sample, by first sonicating 1 mg of the active material in 10 mL of acetone for 30 min. The sample was drop-cast over a carbon coated copper grid of 3.05 mm diameter and dried for 30 min. for solvent evaporation, and then used. The UV-Vis absorption spectrum of a solution of PCZ was measured on a Shimadzu UV-3600 spectrophotometer. Fourier transform infrared (FTIR) spectrum of PCZ was recorded on an Alpha-BRUKER spectrometer in the wavenumber range of 4000-400 cm-1. X-ray photoelectron spectroscopy (XPS) experiments were performed on an ESCA+ Oxford Instruments system functioning at a base pressure of ~5 × 10-11 Torr with a MgKα source at 1253.6 eV. The survey spectra were acquired with 100 eV pass energy with a resolution of 1 eV. The core level spectra were deconvoluted using a non-linear iterative least squares Gaussian fitting procedure after appropriate background subtraction. Corrections due to charging effects were made by using C (1s) as an internal reference and the Fermi edge of a gold sample placed in electrical contact with the sample. A Jandel Peak FitTM (version 4.01) program was used for the analysis. Results and discussion XRD studies and I-V characteristics XRD patterns of CCNFs, pristine Se and Se@CCNFs-20 hybrid are shown in Figure S1a (supporting information). CCNFs show a highly intense peak at 2θ = 26.1°, followed by two broad low intensity peaks at 43.8° and 53.4o. The three peaks correspond to the (002), (101) and


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(004) planes with interplanar spacings of d = 3.35, 2.04 and 1.85 Å of hexagonal graphite (JCPDS: 751621). A broad medium intensity peak is observed at 12.1° corresponding to an interplanar distance of 7.44 Å; this peak arises due to the interplanar separation enhanced by oxygen functionalities which are bonded to the carbon atoms. CCNFs thus are composed of graphitic carbons and the carbons with oxygen containing functional groups. Pristine Se and the Se@CCNFs-20 hybrid show peaks at 2θ = 23.5°, 29.6°, 41.3°, 43.8°, 45.3°, 51.6°, 55.9°, 61.2° and 65.3° that align with the (100), (101), (110), (012), (111), (201), (003), (022) and (120) planes with d =3.78, 3.0, 2.18, 2.07, 1.99, 1.76, 1.65, 1.50 and 1.42 Å of the hexagonal crystal structure of Se (JCPDS: 862246). In the hybrid, an additional peak due to the (002) plane of graphitic carbon from CCNFs is also observed. Electrical conduction of the active materials: pristine Se, CCNFs, PCZ, Se@CCNFs-20 and Se@CCNFs-20-PCZ are compared by recording their I-V characteristics, which are shown in Figure S1b and c. The I-V measurements were performed over a voltage range of −1 to 1V at a scan rate of 10 mV s-1, by sandwiching the samples between two SS substrates as shown in the inset of Figure S1c. The samples were compactly filled in a rectangular cavity having an area of 0.5 cm × 0.2 cm, created on the SS substrate by using a double sided acrylic adhesive tape with (d) 0.64 mm thickness. Se and PCZ show a linear dependence over the entire voltage range, whereas CCNFs, and the Se@CCNFs-20 and Se@CCNFs-20-PCZ show this Ohmic behavior over a voltage range of −0.5 to +0.5 V. Beyond these voltages, the current saturates. From the following expression: R = ρ × d/a or 1/ρ = σ = ∆I/∆V× d/a, where, ∆I/∆V = 1/R or slope of the IV curve, the electrical conductivities are calculated to be 0.013, 11.5, 0.04, 5.6 and 6.8 S cm-1 for Se, CCNFs, PCZ, Se@CCNFs-20 and Se@CCNFs-20-PCZ respectively. CCNFs is composed of dominant graphitic carbons, which are sp2 hybridized, where three electrons from each carbon


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are σ-bonded in the plane and the fourth electron lies in an unhybridized π-orbital that lies above and below the plane, and is free to move across the honeycomb lattice, thus imparting the highest conductivity to CCNFs. The electrical conduction of Se is attributed to its’ lustre, crystalline structure and weakly metallic inter-chain bonding. The electrical conductivity of Se@CCNFs-20-PCZ is higher than that of Se@CCNFs-20, in spite of the lower conductivity of PCZ (0.04 S cm-1). The conductivity of pristine PCZ is measured in bulk, by compacting the polymer in a rectangular cavity. However, in Se@CCNFs20-PCZ, the PCZ layer was applied by a drop-cast method from a clear solution of PCZ in ethanol. After drying, the entire sample was carefully scraped off from the SS substrate and then subjected to the conductivity measurement. In PCZ, the removal of an electron from the π-back bone of the conjugated polymer by doping by a Cl- ion forms a radical cation (polaron), which upon losing another electron forms a bipolaron. Electrical conduction in PCZ, be it bulk or a thin film, is therefore due to the delocalization of positive charges. In a previous report, it has been shown that the energy barrier for inter-chain charge hopping is shown to decrease very significantly after a solution treatment in PEDOT:PSS.20 The conformations of the polymer chains, PCZ, in the present case, change when dispersed in an organic solvent (like EtOH, here), and it is possible that more linear polymer chains as opposed to coiled forms are present in Se@CCNFs-20-PCZ, compared to bulk PCZ. Charge hopping across linear PCZ chains is much easier than across coil conformations. This could be the reason for the increased conductivity of Se@CCNFs-20-PCZ, notwithstanding the lower conductivity of bulk PCZ compared to Se@CCNFs-20. Since electrical conductivity of the three electrodes follows the order: Se < Se@CCNFs-20 < Se@CCNFs-20-PCZ, among the three cells, the PCZ coated electrode is expected to exhibit the


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highest capacity as well as rate performance. Greater the electrical conduction, faster is the electron propagation across the bulk of the electrode during discharge (or charge), and for every electron in the electrode, a charge compensating Li-ion migrates from the electrolyte into the Se electrode, and is thus available for reaction with Se, thus enhancing both capacity and rate performance in contrast to the remaining two electrodes. Electron microscopy studies The SEM image of CCNFs (Figure 1a) show networks of interlinked carbon nanofibers, which are a few microns long. The high magnification image (Figure 1a′) shows discrete fibers which are randomly oriented; they are curved, bent and straight. Pristine Se is characterized by irregular shaped particles with a wide variation in their sizes, from 1.5 to 5 µm in dimensions (Figure 1b). Since the particles are aggregated, when pristine Se is used as an electrode in a Li-Se cell, it is most likely that only the surface sites will be accessible for electrochemical reaction with Li-ions during discharge, thus preventing the full utilization of Se material, and lowering capacity. The enlarged view of an aggregate in Figure 1b′ displays the flaky nature of Se. In the hybrid, the Se particles are uniformly superimposed over the carbon nanofibers (Figure 1c). Se particles are inhibited from agglomerating by the carbon nanofibers in the hybrid, and therefore the Se@CCNFs-20 hybrid is expected to deliver a high Li-ion storage capacity, for more number of Se sites will be available for Li2Se formation compared to pristine Se. The magnified view of the hybrid (Figure 1c′) reaffirms that the agglomeration of Se particles is inhibited to a great extent in the hybrid by the CCNFs. Upon PCZ coating over the surface of hybrid, the morphology of the resulting Se@CCNFs-20-PCZ hybrid (Figure 1d,d′) is characterized by two features which distinguish it from the Se@CCNFs-20: (i) the particle density distributed over the carbon nanofibers is significantly enhanced (compared to Se@CCNF-20), and (ii) the overall contrast in


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the images is relatively fuzzy. This indicates that PCZ is composed of irregular shaped particles, which are superimposed over a sheet like structure. The PCZ coating is better perceived in the TEM images. PCZ not only coats but also acts as a conductive glue that links the Se particles and improves the charge transport between the Se particles. The TEM image of CCNFs (Figure 2a) shows a few discrete carbon nanofibers, and these nanofibers are bulgy at different uniformly spaced regions across the length of a given fiber. The carbon fibers are cone-shaped in the intermittent regions of the fiber, which is clearly evident in the micrograph. A high resolution image of a carbon fiber is displayed in Figure 2b; the walls are crystalline with lattice fringes oriented along the (002) plane and a magnified view is shown as an inset of Figure 2a. The walls are approximately 240 nm thick, and the inner diameter is about 176 nm of each fiber. The selected area electron diffraction (SAED) pattern of the fiber is shown in Figure 2c, which is made up of diffuse rings with bright spots. The spots are assigned to (002), (112) and (103) planes matching with inter-planar spacings of 3.39, 1.1 and 1.5 Å of graphitic carbon. The TEM images of pristine Se show clusters of Se particles of indefinite shapes and their sizes range from 50 to 100 nm (Figure 2d and e). It is these clusters which form the very large grains that are observed in the corresponding SEM image. The SAED pattern of Se (Figure 2f) is fuzzy and reveals two spots originating from the (220) and (201) planes that concur with d spacings of 1.09 and 1.7 Å of Se with a hexagonal crystal structure. The TEM image of the Se@CCNFs-20 hybrid (Figure 2g) shows Se particles having diameters varying from 160 to 400 nm flanked to the carbon nanofibers. The distribution appears to be very homogeneous which is most beneficial for maximizing the Li-ion uptake. While it is apparent that Se particles are outside of CCNFs, but they are anchored to CCNFs, via van der Waals interactions. During discharge, electrons can not only easily move through the tubular structures afforded by the


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CCNFs, but owing to the direct intimate contact that Se particles have with CCNFs, they are rapidly transferred from CCNFs to the Se particles. Further, the voids between the fibers can trap the polyselenides during discharge, and the CCNFs can also weakly bind to the polyselenides via van der Waals’ interactions, and aid in preventing them from diffusing away. The superior Li storage capacity of Se@CCNFs-20 hybrid compared to pristine Se, confirms that the electrical conductivity of CCNFs and the void spaces provided by the CCNFs contribute to preserving a reasonably high capacity. Previous reports support this claim. Hu et al.,21 reported a carbon-sulfur composite with porous hollow carbon nanocapsule monolith to improve the performance of Li-S battery. This composite showed good cycling stability as the porous hollow carbon nanocapsule with nano-sized internal void spaces for facilitating ion and electron transfer and suppresses the diffusion of polysulfides. In another report by Manthiram’ group,22 a high sulfur loading electrode (MHSE) structure improved the sulfur utilization along the fibers of MWCNT, and sulfur also filled in the voids formed in between the fibers thus allowing more S to participate in the reactions which increased the reversible capacity. Mi et al.23 reported nanostructured carbon as anode for a Li-ion battery, which consisted of vertical carbon nanotubes on few layers of graphene. The voids between the vertical carbon nanotubes provided shorter pathways for both electron and ion transport, and the defects in nanotubes provided more active sites for Li-ions, and these factors cumulatively enhanced the electrochemical performance. Also these voids,24 improved the electrochemical performance by accommodating the huge volume expansion thereby relieving the stress. The lattice scale image of the hybrid (Figure 2h) reveals lattice fringes from Se crystallites overlapping with those of CCNFs, indicative of a nano-level contact between the two entities. The spots in the SAED pattern (Figure 2i) are indexed as (100) and (302) planes that align with d


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= 0.37 and 1.13 nm of hexagonal Se, and a spot is matched to the (112) plane corresponding to an inter-atomic distance of 0.12 nm in graphite. The TEM image of pristine PCZ (Figure 2j) reveals the polymer to be composed of particles that aggregate to give a sheet-like appearance. TEM images of the Se@CCNFs-20-PCZ hybrid (Figure 2k,l) reveal that PCZ is coated over the Se particles and CCNFs. PCZ attaches (non-covalently, due to the functional groups) to the polyselenides during cycling thereby reduces the dissolution of polyselenides in the electrolyte. Both PCZ and CCNFs play important roles in improving the Li-ion storage capacity and cycling stability of Li-Se cells. They serve as conductive interconnects thereby providing facile pathways for fast electron and ion transport through the cathode. XPS studies The survey spectrum of the Se@CCNFs-20-PCZ hybrid is shown in Figure 3a. It shows several peaks located at 53.8, 199.6, 282.8, 399.2 and 531 eV, and they correspond to the Se3d, Cl2p, C1s, N1s and O1s levels. While the peak corresponding to Se3d arises from Se powder, the C1s and O1s signals originate largely from CCNFs. PCZ also contributes to the C1s signal, but its’ presence is fully confirmed from the presence of the N1s and the Cl2p signals. “Nitrogen” is present only along the back bone of the PCZ chains in the form of –NH- groups, and the Cl2p signal stems from the chloride ions that are used for doping the PCZ polymer (as can be seen in Scheme 1). The core level C1s spectrum (Figure 3b) is deconvoluted into two peaks located at 284.2 and 285.2 eV corresponding to the C-C (from CCNFs and PCZ) and C-OH (from CCNFs) bonds respectively. The -OH groups are attached to the surface of CCNFs. Figure 3c shows the N1s spectrum, and the two fitted peaks are centered at 399.2 and 401.2 eV. These are due to the C-N and -NH- bonds in PCZ, thus ratifying that the polymer is present in the hybrid. In Figure 3d,


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two fitted peaks of Se3d at 54.4 and 55.2 eV correspond to the Se 3d5/2 and Se 3d3/2 components due to the spin-orbit coupling. The O1s peak (Figure 3e) of the hybrid can be fitted into two peaks are observed at 531.4 and 532.7 eV. These are assigned to C-OH and adsorbed water respectively. The Cl2p peak (Figure 3f) is fitted into two peaks at 198.1 and 199.7 eV, due to the Cl2p1/2 and Cl2p3/2 components. These peaks are from the chloride ions which prevail along the back bone of the PCZ chains, as dopant ions. The XPS analysis confirms existence of PCZ, Se and CCNFs in the hybrid. The presence of the NH groups and Cl- ions, show that PCZ exists in the oxidized (or doped) state in the hybrid, and therefore radical cations are present along the polymer chains. Electrochemical stability tests (discussed later) confirm that PCZ plays crucial role to gain long term cycling stability and high rate capability. The presence of Cl- ions in the Se@CCNFs20-PCZ hybrid, show that PCZ exists in the oxidized (or doped) state in the hybrid, and therefore radical cations are present along the polymer chains. This positively charged PCZ backbone plays an active role in binding the anionic polyselenides during discharge, via electrostatic attractive forces. Since the forces are non-covalent, we conjecture that they are strong enough to confine the polyselenides during discharge and weak enough to release the polyselenides during charge. For had this not been the case, the Li-storage capacity and electrochemical cycling stability of the Se@CCNFs-20-PCZ hybrid would not have been superior to that of Se@CCNFs20 (without PCZ). Unravelling the properties of the PCZ coating The SEM images of PCZ (Figure 4a and b) reveal that the conducting polymer exists in the form of clumps composed of short fibrillar shapes and stub like structures which are compactly packed. The fibers are 0.2 to 1 µm long and they are 100 to 300 nm wide. The high conductivity


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of PCZ can also ascribed to these short fibers, as electron movement is easier through such 1D structures as opposed to misshaped particles. This is the macroscopic view of the polymer, and therefore it differs from the enlarged view observed in the TEM image. The XRD pattern of PCZ (Figure 4c) exhibits prominent diffraction peaks at 2θ = 9.4, 18.6, 20.1, 24.2, 26.9, 27.65 and 31.6o and they match with d = 0.94, 0.48, 0.44, 0.37, 0.33, 0.32 and 0.28 nm of carbazole with a orthorhombic structure (JCPDS: 32-1556). These peaks are indexed to the (020), (040), (111), (131), (141), (201) and (002) planes. However, the observed d-values are slightly shifted compared to that of carbazole, and three additional peaks are also observed at 12.5 (0.71 nm), 15.5 (0.57 nm) and 16.7

o

(0.53 nm), which indicate the formation of the polymer. The UV-

visible spectrum of PCZ in ethanol was recorded in the wavelength range of 200-700 nm (Figure 4d). A highly intense absorption band is observed at 296 nm with a shoulder at 267 nm which are due to the electronic transitions from the bonding (π) 19and non-bonding (n) orbitals to the antibonding orbital (π∗) of the nitrogen hetero-atom, labelled as π→π∗ and n→π∗ transitions.25 PCZ is green in color and it also shows a broad absorption band in the visible region with a λmax at 476 nm. The absorption edge is observed at 638 nm, and the band gap by using the relation: Eg (eV) = 1240 / λ (nm), is estimated to be 1.9 eV, indicating that PCZ is semiconducting and this property is advantageous for fast ion transfer at the Se@CCNFs-20-PCZ interface. To ascertain the formation of PCZ, the FTIR spectrum of PCZ was recorded and it is provided in Figure 4e. The spectrum shows multiple strong peaks at 673, 795 and 874 cm-1, which are assigned to the out of plane bending modes of C-N, C-H and N-H bonds. Distinct bands are further observed at 1090, 1188, 1239, 1305, 1450 and 1600 cm-1 which are attributed to the C-CC trigonal bending, in-plane C-H bending, in-plane N-H bending, C-N stretching, C-C stretching and carbazole ring vibration modes. A broad band is further observed at 3272 cm-1 which arises


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from the N-H stretching mode. While these peaks are assigned to the carbazole structure,26 but these bands are downshifted compared to pristine carbazole, suggestive of the polymerized form of carbazole. The C-C and C-N bond strengths decrease upon polymer formation, and therefore the frequencies of the peaks corresponding to the vibrations of these bonds shift to lower wavenumbers or energies as observed here. In the Li-Se cell, PCZ functions as the interfacial layer that not only prevents the polyselenides from detaching from the current collector and dissipating into the electrolyte, but it also allows Li-ions to easily interact with the Se sites. For PCZ to be an efficient interfacial layer, it is imperative for PCZ to have good electrochemical activity. To evaluate the ability of PCZ to serve as an electroactive layer, a cyclic voltammogram of a PCZ film was measured in a 1 M LiN(CF3SO2)2 based liquid electrolyte (Figure 4f). The film shows a broad oxidation peak at +0.41 V in the anodic branch, and in the reverse sweep, a broad reduction wave is observed, and these features are attributed to the doping and de-doping of the polymer by the imide ions. The enclosed integrated area under the CV curve is 0.75 mA.V and it further evidences the ability of PCZ to work as an effective interfacial layer during repetitive battery cycling. Cyclic voltammetric studies on Li-Se cells Figure 5 shows the initial ten cycles of cyclic voltammetry for Li-Se, Li-Se@CCNFs-20 and LiSe@CCNFs-20-PCZ cells measured over the voltage range of 1.65 to 3 V at a sweep rate of 0.2 mV s-1. All the three cells exhibit two pairs of reversible redox peaks representative of the multiple phase change reversible reactions between Se and Li2Se.27 In the first discharge, pristine Se undergoes reduction to form the insoluble Li2Se/Li2Se2 via the soluble Li2Sen (n ≥ 3) species, and these processes are reflected in the form of a single cathodic peak at 2.05 V. In the reverse sweep however, the oxidation of Li2Se to Li2Sen and then subsequently to elemental Se are


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evidenced from the two peaks at 2.21 and 2.29 V respectively. This is consistent with the delithiation mechanism of the Se cathode in ether-based electrolyte reported by Cui et al.28 With cycling, the peaks intensities fade- and peak positions shift- progressively. The reduction peak shifts to a more positive voltage of 2.19 V and the oxidation peaks shift to 2.23 and 2.33 V respectively in the 10th cycle. The cells with the Se@CCNFs-20 and Se@CCNFs-20-PCZ cathodes demonstrate altered reduction profiles: a sharp reduction peak at 2.08/2.09 V due to Se → Li2Sen reaction, followed by a broad peak at 1.89/1.86 V due to Li2Sen → Li2Se2/Li2Se reaction, are observed in the first discharge. The fact that the two processes are distinctly perceptible through an electrochemical probe, is because in the hybrid cathodes, the volume expansion that Se undergoes to form Li2Se is better buffered due to CCNFs, and this manifests itself in the form of two resolved peaks instead of one as in pristine Se. Oxidation peaks at 2.21/2.22 V and 2.27/2.27 V correspond to the reversible oxidation of Li2Se to polyselenides and Se respectively for the two cells during the charging process. In the first cycle, the pristine Se cell exhibits a single cathodic peak at 2.05 V with a peak current of 1.59 mA and two oxidation peaks at 2.21 and 2.29 V with 1.41 and 0.96 mA currents respectively. Both Li-Se@CCNFs-20 and Li-Se@CCNFs-20-PCZ cells exhibit two reduction peaks at 2.08/2.09 V with a peak current of 2.77/0.77 mA followed by a broad peak at 1.89/1.86 V with currents of 1.03/ 0.62 mA. Two oxidation peaks at 2.21/2.22 V and 2.27/2.27 V with 4.01/3.15 and 1.73/0.94 mA currents during cathodic and anodic scans respectively are observed. The 1st cycle usually shows an anomalous response, and ideally should not be used for comparing different electroactive materials. The Li-Se@CCNFs-20-PCZ cell exhibits low anodic and cathodic peaks current compared to the Li-Se@CCNFs-20 cell which is probably due to the insufficient diffusion of Li-ions and electrolyte initially in the PCZ based hybrid. One more


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difference is that the fading of anodic and cathodic peak current decreases with cycling after 2nd cycle in the Li-Se@CCNFs-20-PCZ cell. In contrast, for the Li-Se@CCNFs-20 cell, slightly faster decay of peak current occurs. This is reflected in the cycling stability. But in case of the Li-Se cell, the peak current drastically fades with cycling. Due to this reason the CV curves are dissimilar for Li-Se and hybrid based Li-Se cells. On comparing the CV curves of the hybrid based cells from the 2nd cycle onwards, the reduction peaks are shifted to more positive voltages of 2.23/2.23 V with currents of 0.60/0.48 mA and to 1.77/1.91 with currents of 0.71/0.48 mA in the 10th cycle. This indicates there is an electrochemical activation process of Se and improvement in the reversibility of hybrid based LiSe cells during lithiation.29 But, in the case of Li-Se cell, the peaks do not shift and at the same time, fast fading of peak current occurs with cycling which is also reflected in the poor capacities observed during cycling. Among the 3 cells, the Se@CCNFs-20 hybrid based cells, and particularly, the one with the PCZ coating furnishes a stable reproducible reversible redox response, with minimum variation in the CV profile after the 6th cycle. The position of the cathodic peak is controlled by the length of the amorphous Sen chains involved in the lithiation process.8 A shift of the cathodic peak potential to higher positive values, implies a longer Sen chain with more Se atoms. Thus from the 6th cycle onwards, since the cathodic peak position remains unchanged, the Sen chain length is stabilized and the oxidation-reduction process is fully reversible for the PCZ based cell. The CV studies, thus bring to the fore, the role of PCZ in endowing the Se@CCNFs-20 cathode with redox stability, a pre-requisite to ensure long operational life to the Li-Se battery. Optimization of CCNFs content in the Se@CCNFs hybrid


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In the previous sections, the Se@CCNFs-20 hybrid was used for all characterizations, and it was based on the ability of this hybrid to deliver the highest initial- and reversible- capacities in comparison to the Se@CCNFs-10 and Se@CCNFs-30 hybrids, when used in Li-Se cells. Experimental validation for the use of the Se@CCNFs-20 hybrid is accomplished by comparing the 1st, 2nd and 3rd galvanostatic charge-discharge curves for the Li-Se@CCNFs-10, LiSe@CCNFs-20 and Li-Se@CCNFs-30 cells recorded at 0.1 C-rate within the voltage range of 1.65-3 V. The plots are shown in Figure 6a-c respectively. This limiting voltage of 1.65 V is imposed by the electrolyte (1 M LITFSI and 0.2 M of LiNO3 in DOL/DME), that is employed in the cells. By using this electrolyte, when a Li-Se cell is discharged to voltages lower than 1.65 V, the capacity and cyclability of Li-Se battery are adversely affected. This is due to the irreversible reduction reaction of Se with LiNO3 at voltages lower than 1.65 V, and similar reactions have been unambiguously confirmed in the past for Li-S batteries.30,31 On the other hand, we find that we cannot go above 3 V, during the charging cycle, because, it is evident from Figure 6 that over the higher voltage range of 2.3 to 3 V, a steep straight line behavior is observed for all the cells, which indicates that there is no further chemical reaction or oxidation in this range, and therefore, it is natural to conclude that only some decomposition current will exist at voltages > 3 V. Furthermore, CV results, which are presented in Figure 5, corroborate this inference. No peaks are observed at voltages greater than 2.4 V during the anodic scans in the CV plots. Therefore, the voltage range of 1.65 to 3 V is an appropriate voltage window for a long lived LiSe battery containing an ether based electrolyte. In this study, the specific capacities are calculated on the basis of mass of Se present in the relevant working electrode or cathode and their galvanostatic charge-discharge curves were measured 0.1 C-rate (67.8 mA g-1). The initial charge and discharge capacities of Se@CCNFs-


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10, Se@CCNFs-20 and Se@CCNFs-30 hybrid based cells with 70, 60 and 50 wt% of Se are 552.9, 990.3, 443.8 mAh gSe-1 and 664.8, 806.2, 378.5 mAh gSe-1 respectively. The cycling stabilities of the Li-Se@CCNFs-10, Li-Se@CCNFs-20 and Li-Se@CCNFs-30 cells measured at 0.1 C-rate for 40 cycles are compared in Figure 6d. Reversible capacities of 79.9, 639.5 and 247 mAh gSe-1 are retained for Se@CCNFs-10, Se@CCNFs-20 and Se@CCNFs-30 hybrid cells at the end of 40 cycles. The initial capacity is the largest for the hybrid cell with 60 wt% of Se, because at a higher Se loading, as in the cell with 70 wt% of Se, the Se-particle aggregation is foreseen to be higher than that expected in cells with lower Se loadings of 60 and 50 wt%. Consequently, though the amount of Se physically present in the Li-Se@CCNFs-10 cell is higher than that in the other two cells, but the amount of active Se available for electrochemical reaction with Li-ions is reduced by the uncontrolled aggregation, thus resulting in a first discharge capacity lower than the one with 60 wt% of Se. With cycling, this cell with 70 wt% of Se is worst affected due to the insufficient amount of the CCNFs (only 10 wt%) available to trap the polyselenides during cycling. This leads to disintegration of Se and its’ removal from the current collector, thus causing a huge drop in charge capacity, by ~85.6% after 40 cycles. Of the 3 cells, the LiSe@CCNFs-30 cell shows the lowest initial capacity, due to a lower Se content in the cathode (compared to the 60 and 70 wt% Se cells). But the capacity retention with cycling is improved for this cell relative to the cell with 70 wt% of Se. This is due to the large amount of CCNFs (30 wt%) available at the cathode which bind to the polyselenides weakly and prevent their loss via cross-over and dissolution. Hence, the reversible capacity reduces by 44.3% after 40 cycles. However, the Li-Se@CCNFs-20 cell outperforms both the Li-Se@CCNFs-10, and LiSe@CCNFs-30 cells in terms of superior- initial capacity and capacity retention with cycling. At


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60 wt% Se and 20 wt% CCNFs loadings, the Se particles aggregate to a lesser extent due to an adequate CCNFs content, thus maximizing Se utilization and leading to the highest initial capacity. Simultaneously, the CCNFs also restrict the loss of active Se through the migration of soluble polyselenides thereby resulting in an enhanced capacity retention. After the completion of 40 cycles, the charge capacity for the Li-Se@CCNFs-20 cell is decreased by only ~35.4%. Furthermore, to study if Se@CCNFs prepared by a heat treatment can yield better performance than the unheated material, a Se@CCNFs hybrid was prepared by a melt diffusion method. Se powder and CCNFs mixture was heated at 260 oC for 12 h, and then upon cooling, the resulting hybrid delivered a lower performance than the hybrid prepared by dry-grinding. The initial capacity is 617 mAh gSe-1, which reduces to 294 mAh gSe-1, after 80 cycles (Figure S2, supporting information). The poor performance might be due to the agglomeration of Se particles in the hybrid. The advantages of the dry grinding method over the melt diffusion method are as follow. (1) In the melt-diffusion method, it is not possible to ensure a uniform dispersion of Se particles in the CCNF based matrix. (2) Also, it is not possible to precisely control the amount of Se that will impregnate in CCNFs, using the melt diffusion method. In contrast, in the dry grinding method, the exact amount of Se can be weighed and mixed with the carbon matrix, and therefore capacities can be determined with a higher degree of accuracy. (3) Further, the dry grinding method causes minimum damage to the Se particles and CCNFs; their individual structural integrities are maintained in the hybrid and therefore high electrical conductivities are preserved. (4) The dry grinding method has no any influence on the porous structure, volume and surface area of CCNFs. These results thus indubitably justify the use of the Li-Se@CCNFs-20 (dry-grinded) cell for practical applications.


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CCNFs and PCZ control electrochemical performances of Li-Se cells Galvanostatic charge-discharge voltage profiles for the Li-Se, Li-Se@CCNFs-20 and LiSe@CCNFs-20-PCZ based cells measured intermittently from 1st to 100th cycle at 0.1 C-rate over a voltage range of 1.65-3 V are displayed in Figure 7a-c. The 3 cells under study exhibit two well-defined plateaus during discharge due to the lithiation of Se to soluble polyselenides and insoluble Li2Se2 and Li2Se. Upon charging, only one plateau is observed due to the delithiation of Li2Se to soluble polyselenides and Se. The charge-discharge voltage profiles are almost identical to those observed in reports where ether based electrolyte is used

13,19

but

different from the studies using carbonate based electrolyte.17,32 Cells with pristine Se, Se@CCNFs-20 and Se@CCNFs-20-PCZ cathodes show initial discharge capacities of 225.6, 806.2, 885.5 mAh gSe-1 and charge capacities of 288.9, 990.3, 1076.5 mAh gSe-1 with Coulombic efficiencies of 78, 81.4 and 82.3% respectively. The charge-discharge voltage plateaus remain stable even after 100 cycles for the cells containing the CCNFs hybrid. The initial capacity is the highest for the cell where PCZ is coated over the Se@CCNFs-20 hybrid. PCZ provides an electrochemically active interface, and serves as an additional barrier to restrict the dissolution and shuttle of polyselenides, thus increasing the first capacity. Further, the electrical conductivity of the Se@CCNFs-20-PCZ is 1.2 times greater than that of Se@CCNFs-20, which promotes electron movement, and increases Li-ion uptake. Pristine Se based cell shows the lowest capacity due a very high Se loading (80 wt%) The uninhibited coalescence of Se particles, and the lower electrical conductivity of pristine Se (523 times lower than that of Se@CCNFs-20-PCZ!) result in poor Se utilization and thus low capacity. The voltage gap between the charge and discharge voltage plateaus for the hybrid based Li-Se cells is smaller than that of pristine Se based cell. This indicates low polarization,


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implying electrochemical stability and good reversibility for the Se@CCNFs-20 and Se@CCNFs-20-PCZ based cells. According to Figure 7d, the Li-Se@CCNFs-20 and Li-Se@CCNFs-20-PCZ cells show initial discharge and charge capacities of 806.2 and 990.3 mAh gSe-1, 885.5 and 1076.5 mAh gSe-1 respectively, which are greater by 18.9% and 46.1%, 30.6% and 58.8% than the theoretical capacity of Se i.e., 678 mAh gSe-1. Similarly in Figure 7e, the Li-Se@CCNFs-20 and LiSe@CCNFs-20-PCZ cells show initial discharge and charge capacities of 1071 and 876 mAh gSe1

, 862 and 835 mAh gSe-1 respectively, which are also greater by 58% and 29.2%, 27.1% and

23.2% than the theoretical capacity of Se. This is due to (1) Li-ion trapping in the CCNFs,15, 33-35 (2) parasitic reactions of Se with Li-ions and (3) the decomposition of electrolyte. However, we also showed that pristine CCNFs contribute insignificantly, over long term cycling, to the Listorage capacity by studying Li-blank CCNF cells; details are provided in supporting information (Figure S3). After 100 cycles, the cells with the Se@CCNFs-20 and Se@CCNFs-20-PCZ cathodes retain reversible capacities of 531.6 and 640.3 mAh gSe-1 with 53.6 and 59.4% capacity retention. In great contrast, is the performance of the Li-Se cell with pristine Se. The first charge and discharge capacities are 288.9 and 225.6 mAh gSe-1 and after 100 cycles, they reduce to 123.6 and 121.2 mAh gSe-1 in the 100th cycle at the rate of 0.1 C. It is apparent that the PCZ coating plays an active role in improving the first capacity and it also assists the cell in minimizing the cycling induced capacity fade. In the Li-Se@CCNFs-20 cell, the capacity decay is slow at the beginning. In the 11th cycle, the reversible capacity stabilizes to 801.71 mAh gSe-1. Thereafter, the capacity decay per cycle is 0.03% for the next 90 cycles. The capacity decay rate for the Li-


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Se@CCNFs-20-PCZ cell is extremely low after 10 cycles. The reversible capacity is 686.12 mAh gSe-1 in the 11th cycle, and the decay rate is 0.0051% per cycle over the subsequent cycles. We also find that the capacity of the Li-Se@CCNFs-20-PCZ cell is greater than that previously reported for many Se/C cathodes used in Li-Se batteries and operated at 0.1 C-rate. A Se/CNT hybrid10 delivered a capacity of 400 mAh gSe-1 which dropped to 315 mAh gSe-1 after 100 cycles. Another report on a Se impregnated in a nitrogen doped carbon scaffold or a Se/NCSs composite 36

with 56 wt% of Se initially showed a capacity of 960 mAh gSe-1 which decreased to 480 mAh

gSe-1 in the 100th cycle. A Se/MCN-RGO composite37 with 62 wt% of Se delivered a reversible capacity of 568 mAh gSe-1 in the 100th cycle. For a PANI@Se/C-G composite38 with 51.9 wt% of Se, capacity reduces from 628 to 588.7 mAh gSe-1 in the 200th cycle. For a Se/CMK-3 porous carbon hybrid, an initial capacity of 900 mAh gSe-1 decreased to 600 mAh gSe-1 after 50 cycles at 0.1 C-rate.39 In another study,16 Zhang et al., improved the capacity of a Li-Se cell by inserting a carbon interlayer between cathode and the separator; the capacity declined from 656 to 520 mAh gSe-1 in 20th cycle. In comparison with this last report where a carbon interlayer was used to improve capacity, here the PCZ coating appears to be much more effective in regulating the capacity fade. Besides controlling the initial capacity and operational lifespan, the PCZ coating also governs the rate performance favorably. The C-rate was varied from 0.1 to 2 C within the voltage range of 1.65 to 3 V and back to 2 C, and the performances are compared for the Li-Se@CCNFs-20 and Li-Se@CCNFs-20-PCZ cells in Figure 7e. As the C-rate is increased systematically from 0.1 to 0.2, 0.3, 0.5, 1 and 2 C, the Li-Se@CCNFs-20-PCZ cell shows a charge capacity varying from 1071 mAh gSe-1 to 482, 427.7, 338.6, 205.6 and 110.8 mAh gSe-1 respectively. When the C-rate was switched back from 2 to 0.1 C, the cell recovers charge capacities of 202.5, 254.9, 346,


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391.3 and 548 mAh gSe-1 respectively after the high rate cycling test. Without the PCZ coating, the Li-Se@CCNFs-20 cell shows a comparatively inferior rate performance. It delivers a charge capacity of 983.66 mAh gSe-1 at 0.1 C-rate, but as the rate increases from 0.1 to 2 C, capacity decreases dramatically to 22.7 mAh gSe-1. After the high rate measurements, a reversible low capacity of 497 mAh gSe-1 was recovered when the C-rate was restored to 0.1 C. The higher electrical conductivity of the Se@CCNFs-20-PCZ cathode, the ability of PCZ to function as an interfacial layer conducive for the to- and fro- movement of Li-ions, and that it can also serve as a physical barrier to block the polyselenides are the reasons for fast and efficient kinetics. The Li-Se@CCNFs-20 cell initially shows a performance almost similar to that of the LiSe@CCNFs-20-PCZ cell at 0.1 C-rate. But, the same Li-Se@CCNFs-20 cell shows poor rate performance at higher C-rates compared to the Li-Se@CCNFs-20-PCZ cell. The poor rate performance of the Li-Se@CCNFs-20 cell is due to lower electrical conductivity of Se@CCNFs20 compared to the same electrode coated with PCZ. The superior electrical conductivity, facilitates faster electron and ion transport, particularly, at high C-rates, and thus improves the reaction kinetics upon cycling. Another advantage of the PCZ layer, is that it can trap polyselenides and therefore more polyselenides can be converted into Li2Se. This is also evidenced from the very low capacity decay of 0.0051% per cycle, achieved during cycling in this cell. The Coulombic efficiencies (CEs) of Li-Se, Li-Se@CCNFs-20 and Li-Se@CCNFs-20-PCZ cells are shown in Figure 7f. The CE of Li-Se, Li-Se@CCNFs and Li-Se@CCNFs-PCZ based cells in the first 40 cycles lies in the range of 78-96%, 74-96% and 82-96% respectively. At the end of 100 cycles, the CEs approach 98%, 96.9% and 98% for Se, Se@CCNFs-20 and Se@CCNFs-20-PCZ based cells, indicating that the polyselenide crossover is reduced.40,41


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CEs are low in the first few cycles due to the following reasons. (i) The solid electrolyte interphase (SEI) is formed in the initial cycles, which traps Li-ions irreversibly and (ii) the possibility of Li-ion trapping by CCNFs. This is commonly found in Li-Se battery systems. In a previous study on a Se@CNFs-CNT composite,42 the first CE is 50.9%; authors propounded that the low initial CE is due to the formation of an irreversible SEI. In another study, for a TiO2-Se composite cell, CE was observed to vary between 50-60%, and it stabilized only after ~30 cycles.15 Similarly, for a cell with a Se/interconnected porous hollow carbon bubbles composite cathode, in the 1st cycle, CE was less than 80%, and it vacillated with cycling, with an appreciable drop for a few cycles at around ~80 cycles.34 SEI, shuttle reaction, formation of Li2Se on the surface of Li anode

32

and the tendency of porous carbon nanomaterials to trap Li-

ions are the plausible reasons for the lower CEs in the first few cycles in Se/C composites.15,33,34 The Li-Se@CCNFs and Li-Se@CCNFs-PCZ based cells retain reversible capacities of 251 and 471 mAh gSe-1 after 300 cycles (inset of Figure 7f). These results have also been compared with previous literature reports in supporting information (Table S1), and from the table it is deduced that the Li-Se@CCNFs-PCZ cell outperforms most of the reported Se composites, at 0.1 C-rate and 60 wt% loading. The retained capacities are 37% and 70% of theoretical capacity of Se. This clearly demonstrates that the PCZ coating enhances the cycling stability by confining the polyselenides at the cathode, effectively inhibiting their dissolution in electrolyte and restricting their shuttle, in striking contrast to the Li-Se@CCNFs cell. In the latter, although the CCNFs do contain the polyselenide loss, but they are not as effective as the combination of CCNFs and PCZ. Post-mortem analysis


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To analyze how the cathodes endure long term cycling, the SEM images and XRD patterns of the Se, Se@CCNFs-20 and Se@CCNFs-20-PCZ cathodes were recorded after they had been subjected to 100 charge-discharge cycles at 0.1 C-rate (Figure S4, supporting information). All electrodes shows peaks at 2θ = 50.7˚ and 64.7˚ matches with d = 1.76 and 1.42 Å corresponding to the (201) and (120) planes of hexagonal crystal structure of Se (Figure S4a). In addition the pristine Se electrode shows extra peaks at 2θ = 23.3˚, 29.9˚and 47.9˚ corresponds to the (100), (101) and (200) planes of Se. The electrodes become increasingly amorphous with cycling. The Se electrode (Figure S4b) shows large voids, indicative of the detachment of active material from the current collector. The Se@CCNFs-20 and Se@CCNFs-20-PCZ cathodes (Figure S4c and d) show a fibrous morphology, due to electrolyte decomposition on their surfaces. No large gaps are visible, suggestive of the fact that the electrodes sustain cycling without significant material loss, which is also reflected in their post-cycling reversible capacities. To confirm the form in which Se exists after cycling, Raman spectra of Se@CCNFs-20 and Se@CCNFs-20-PCZ cathodes were recorded before and after they were subjected to 100 chargedischarge cycles at 0.1 C-rate (Figure S5, supporting information). Both the fresh Se@CCNFs20 and Se@CCNFs-20-PCZ electrodes exhibit a peak at 238 cm-1 which is attributed to the A1 symmetric stretching mode of the Se-Se chains. This peak is characteristic of hexagonal Se or crystalline Se, and has been observed in the past at 237.4 cm-1 for hexagonal Se nanowires.43 After cycling, this peak is not observed, and a new peak is observed at ~260 cm-1 for both the electrodes. This peak is characteristic of amorphous or vitreous Se, as per studies reported on Se.43,44 From the Raman studies, it is obvious that Se is largely amorphous after cycling. Conclusions


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In this study, we attempted to address the main hurdles in the advancement of Li-Se batteries by using two simple approaches. (1) By employing CCNFs as the highly conducting matrix: for trapping the polyselenides in their voids or by van der Waals’ interactions thus preventing their dissolution and shuttle, for accommodating the volume expansion during Li2Se formation thus restricting the active electrode pulverization and for fast electron transfer from or to Se during charge and discharge. (2) By applying a coating of PCZ over the SE@CCNFs-20 hybrid cathode, which not only provides an electrochemically active interface for facile reaction of Se with Liions but also functions as an additional barrier to confine the polyselenides to the cathode. Furthermore, other highlights of the Li-Se@CCNFs-20-PCZ cell are the ease of preparation of the cathode material without the use of any high temperature, reduced pressure, multiple tedious steps and capital equipment. PCZ is cheap, is easily synthesizable in bulk quantities, has moderate electrical conductivity and is completely soluble in many organic solvents thus allowing its’ deposition from a clear particulate free solution over the Se@CCNFs-20 hybrid. These characteristics render it to be a practically implementable coating for preserving the high capacity of a Li-Se cell for many cycles. The outcome of this study is a Li-Se@CCNFs-20-PCZ cell with a 60 wt% Se loading. The cell delivers a reversible stable capacity of ~640 mAh gSe-1 at the end of 100 cycles at 0.1 C-rate, shows an exemplary rate performance, a high Coulombic efficiency (~98%) and low polarizability- performance parameters superior to not only (i) other CCNFs based hybrids with lower and higher Se content, (ii) same hybrid but without the PCZ coating and (iii) pristine Se (all prepared in this study) but is also better than many of the previously reported Li-Se/C cells from battery literature. The scalability of the method to fabricate the Li-Se@CCNFs-20-PCZ cell, and its performance serve as the two key solutions for


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addressing the challenges faced by next generation batteries. We believe this study can play pivotal role in shaping the future of Li-Se batteries.

FIGURES:


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Figure 1. SEM images of (a,a′) CCNFs, (b,b′) pristine Se (c,c′) Se@CCNFs-20 hybrid and (d, d′) Se@CCNFs-20-PCZ.


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Figure 2. (a,b) TEM images and (c) SAED pattern of CCNFs; inset of (a) is a lattice scale image of a CCNF wall. (d,e) TEM images and (f) SAED pattern of pristine Se. (g,h) TEM images and (i) SAED pattern of the Se@CCNFs-20 hybrid. TEM images of (j) PCZ and (k,l) Se@CCNFs20-PCZ.

Figure 3. (a) XPS survey spectrum, deconvoluted core level spectra of: (b) C1s, (c) N1s, (d) Se3d, (e) O1s and (f) Cl2p of Se@CCNFs-20-PCZ.


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Figure 4. (a,b) SEM images, (c) XRD pattern, (d) Absorbance versus wavelength spectrum (inset is a magnified view of the visible region) and (d) FTIR spectrum of PCZ. (f) Cyclic voltammetric plot of a SS/PCZ film in an electrolyte solution of 1 M LITFSI + 0.2 M of LiNO3 in DOL/DME (1:1 v/v) with a Pt counter electrode recorded at a scan rate of 5 mV s-1 over a voltage window of −1 to +1 V. In (e): ν: stretching, δ: bending, o: out of plane, i: in-plane and vib: vibration.


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Figure 5. Initial 10 cyclic voltammetry curves for (a) Li-Se, (b) Li-Se@CCNFs-20 and (c) LiSE@CCNFs-20-PCZ cells measured at a scan rate of 0.02 mV s-1 within the voltage range of 1.65 to 3 V.


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Figure 6. Initial 1st, 2nd and 3rd galvanostatic charge-discharge curves for (a) Li-Se@CCNFs-10, (b) Li-Se@CCNFs-20 and (c) Li-Se@CCNFs-30 cells and their (d) cycling stability up to 40 cycles measured at 0.1 C-rate between the voltage range of 1.65-3V; cc and dc represent charge and discharge capacities.


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Figure 7. Galvanostatic charge-discharge curves for (a) Li-Se (b) Li-Se@CCNFs-20 and (c) LiSe@CCNFs-20-PCZ cells from the 1st to the 100th cycle recorded at 0.1 C-rate. (d) Cycling stability at 0.1 C-rate, (e) rate capability at different C-rates and (f) Coulombic efficiency at 0.1 C-rate of different cells over a voltage range of 1.65-3 V; Inset of (f) shows the cycling stability of Li-Se@CCNFs-20 and Li-Se@CCNFs-20-PCZ cell for 300 cycles. In (d) and (e): cc and dc represent charge and discharge capacities.


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Scheme 1. Preparation of Se@CCNFs hybrid, PCZ and the Se@CCNFs-20-PCZ electrode.

SUPPORTING INFORMATION XRD patterns of pristine Se, CCNFs and the Se@CCNFs-20 hybrid and I-V characteristics of active pristine Se, CCNFs, PCZ, Se@CCNFs-20 and Se@CCNFs-20-PCZ. Capacity versus number of cycles for cells with heated and unheated Se@CCNFs based electrodes. Cycling performance of Li-CCNFs cells. Literature survey of the electrochemical properties of Se/C composites. XRD patterns and SEM images of fresh and cycled for Li-Se, Li-Se@CCNFs-20 and Li-Se@CCNFs-20-PCZ cells. Raman spectra of fresh and cycled Se@CCNFs-20 and Se@CCNFs-20-PCZ electrodes. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*Email: [email protected]; Fax: +91-4023016003; Tel: +91-4023016024

ACKNOWLEDGMENT Financial support from the Department of Science and Technology (DST) of India - Science and Engineering Research Board (SERB) (EMR/2015/001775) is gratefully acknowledged. R.M. thanks University Grants Commission (UGC) of India for the award of a senior research fellowship. We thank the TEM facility of IITH. REFERENCES (1) Mi, C. H.; Cao, G. S.; Zhao, X. B. Low-Cost, One-Step Process for Synthesis of CarbonCoated LiFePO4 Cathode. Mater. Lett. 2005, 59, 127-130. (2) Ozawa, K. Lithium-Ion Rechargeable Batteries with LiCoO2 and Carbon Electrodes: The LiCoO2/C System. Solid State Ionics 1994, 69, 212-221. (3) Lee, Y. S.; Sun, Y. K.; Nahm, K. S. Synthesis of Spinel LiMn2O4 Cathode Material Prepared by an Adipic Acid-Assisted Sol–Gel Method for Lithium Secondary Batteries. Solid State Ionics 1998, 109, 285-294. (4) Choi, N. S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges Facing Batteries and Electrical Double –Layer Capacitors. Angew. Chem. Int. Ed. 2012, 51, 9994-10024. (5) Mukkabla, R.; Deepa, M.; Meduri, P.; Shivaprasad, S. M.; Ghosal, P. Sulfur Enriched Carbon Nanotubols with a Poly (3, 4-ethylenedioxypyrrole) Coating as Cathodes for Long-Lasting Li-S Batteries. J. Power Sources 2017, 342, 202-213.


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Poly(carbazole) Coated Selenium@Conical Carbon Nanofibers

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