Graphite Platelet Nanofiber Composite for Durable Li–Se

May 3, 2017 - Long-lasting Li–Se cells with a Se/graphite platelet nanofiber (GPNF) composite is prepared for the first time, and it shows a reversi...
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Selenium/Graphite Platelet Nanofiber Composite for Durable Li−Se Batteries Radha Mukkabla,† Sathish Deshagani,† Praveen Meduri,‡ Melepurath Deepa,*,† and Partha Ghosal§ †

Department of Chemistry and ‡Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Kandi, Sangareddy 502285, Telangana, India § Defence Metallurgical Research Laboratory, Defence Research and Development Organisation (DRDO), Hyderabad 500058, Telangana, India S Supporting Information *

ABSTRACT: Long-lasting Li−Se cells with a Se/graphite platelet nanofiber (GPNF) composite is prepared for the first time, and it shows a reversible capacity of 489 and 384.7 mAh gSe−1 after 200 and 350 charge/discharge cycles, respectively. It shows superior rate capability and low Se polarization even with a high Se (75 wt %) proportion. It also shows higher capacity and better cycling stability compared to conventional Se/carbon material composites (with graphene oxide (GO), reduced GO, and carbon nanotubes). The effectiveness of GPNFs as a conductive support and for inhibiting the shuttle and dissolution of polyselenides in the electrolyte is also confirmed by conducting atomic force microscopy studies. Nanoscale current maps of Se/GPNFs reveal the presence of homogeneously distributed high-current domains, which are retained even after the first discharge. In contrast, the pristine Se electrode is characterized by predominant low-current regions after the first discharge. The ability of GPNFs to enable the preparation of durable and easily processable Li−Se cells is demonstrated.

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In their pioneering effort, Abouimrane and co-workers developed a Li/Se−C cell, and the system sustained a reversible capacity of ∼500 mAh g−1 for >25 cycles at a low current density (10 mA g−1, ∼C/60), which reduced to ∼300 mAh g−1 at a higher current density (50 mA g−1, ∼C/12) with a small fade for 100 cycles.8 Li et al. prepared a Se composite confined within porous carbon nanospheres with a Se loading of 70.5 wt %, and it showed a volumetric capacity density of 3150 mAh cm−3 and an good rate capability (it retained 57% of the theoretical capacity at a 20 C rate).5 They showed cycling stability for 1200 cycles with a capacity decay as slow as 0.03% per cycle. The battery performance was attributed to the high loading of the active material, effective protective film formed on the surface of the active material, and strong adsorbing ability of Se/Li2Sex provided by the micropore-rich carbon structure.5 In a similar vein, metal oxide framework-derived hollow hierarchical porous carbon spheres,9 graphene, carbon nanotubes (CNTs),6 carbon interlayers,10 mesoporous carbon spheres,11 interconnected porous hollow carbon bubbles,12 and carbon/carbide13,14 have also been used to prepare composites with Se, and Li−Se composite cells with better performances compared to those of pristine Se have been reported.

ithium−selenium (Li−Se) batteries are analogues of lithium−sulfur (Li−S) batteries, wherein Se undergoes redox processes similar to sulfur. One selenium atom is capable of reacting with a maximum of two lithium ions (Se + 2Li+ + 2e− ⇆ Li2Se).1 Though lower than that of sulfur, the theoretical gravimetric capacity of Se is calculated to be 678 mAh g−1, which is greater than that of some of the conventional cathode materials that are employed in Li-ion batteries (LIBs) such as LiCoO2 (272 mAh g−1)2 and LiFePO4 (170 mAh g−1).3 Notably, the volumetric capacity of Se (3240 mAh cm−3) is very high, comparable to that offered by S4 (3467 mAh cm−3). The difference is due to the higher density of Se compared to that of S.5 A conspicuous advantage that Se has over S is the much higher electrical conductivity of Se. The electrical conductivity of Se is approximately 20 orders of magnitude greater than that of S.6 Despite the lower theoretical capacity of Se relative to that of S, the higher electrical conductivity of Se (1 × 10−3 S m−1) can result in long cycle life and superior rate performance without the addition of large quantities of conducting carbonaceous compounds. These features render “Se” to be a promising cathode material for rechargeable lithium batteries. However, just like the problems that affect the Li−S battery performance, in the Li−Se cell too, the dissolution and migration of polyselenides in electrolytes during cycling result in fast capacity fading and low Coulombic efficiency, as reported for a nonporous Se cathode.7 © 2017 American Chemical Society

Received: March 22, 2017 Accepted: May 3, 2017 Published: May 3, 2017 1288

DOI: 10.1021/acsenergylett.7b00251 ACS Energy Lett. 2017, 2, 1288−1295

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ACS Energy Letters

graphitized carbons are observed at 1343 and 1572 cm−1, revealing that the GPNF’s matrix is partially graphitized. The morphology of pristine Se (Figure 1a) shows large clusters of Se particles with no regular shape and having

The underlying concept exploited in the above-described works is the use of hollow carbon nano- or microstructures and Se particles that are either tethered to or encapsulated therein to form composites. (i) The high effective surface area of the carbon structure allows maximum loading of Se in the composite, and (ii) the interconnects between the carbon structures impart good electrical conduction, necessary for electrochemical reaction of all of the Se (across the cross section of the material) with Li-ions, and both of these aspects can lead to high capacity. The hollow nature of a given carbon structure (iii) accommodates the volume change that Se undergoes during lithiation and delithiation and (iv) entraps the polyselenides and prevents their diffusion into the electrolyte and their shuttle between the anode and cathode and thus serves to reduce pulverization and active material (Se) loss during the repetitive charge/discharge cycling. While (i)−(iv) attributes have been moderately successful and yielded cells with reasonable cycling stabilities, Li−Se cells, however, are still in their infancy, and there is tremendous scope for modifying the cathode architecture for enhanced and stable electrochemical response, especially with the view of capacity retention with cycling. Here we present a comparative study of a Se/graphite platelet nanofiber (GPNF) composite versus commercial Se as cathodes in Li−Se cells for the first time. We also show that GPNFs are superior to multiwalled carbon nanotubes (MWCNTs) or graphene oxide (GO) or reduced graphene oxide (RGO) and are conductive and buffering supports. With a high Se loading of 75 wt %, this Se/ GPNF composite outperforms most of the Se/C composites reported in the literature on Li−Se cells, in terms of both capacity retention with cycling and rate capability. This report demonstrates that Li−Se cells are promising and more effective substitutes for both Li−S cells and LIBs. The X-ray diffraction patterns and Raman spectra of pristine Se and Se/GPNF composite are shown in the Supporting Information (Figure S1). The diffraction peaks of both materials are indexed with the hexagonal phase of Se (powder diffraction file (PDF): 86-2246). Pristine Se exhibits sharp and well-resolved intense peaks at interplanar spacings (d) of 3.78, 3.0, 2.18, 2.07, 1.99, 1.76, 1.63, 1.50, and 1.42 Å, corresponding to the (100), (101), (110), (012), (111), (201), (112), (022), and (120) planes of the hexagonal crystal structure of Se. The diffractogram of the Se/GPNF composite is almost similar to that of Se except for a singular difference. An additional peak is observed at 2θ = 26.5°, corresponding to d = 3.34 Å, which matches the (200) plane of graphitic carbon from GPNFs, thus ratifying the inclusion of GPNFs in the composite. Because the patterns are nearly identical, it is concluded that no phase transformation occurs during the preparation of the Se/GPNF composite and that the crystallinity of Se is preserved in the composite. In the Raman spectrum, pristine Se exhibits a sharp peak at 235 cm−1, which is attributed to the A1 symmetric stretching mode of Se−Se bonds of Se8 chains in Se.15,13 Two additional peaks are observed at 145 and 457 cm−1, which are attributed to the bending and stretching modes of Se−Se units in Se8 rings.16,17 In the composite, only the peak at 235 cm−1 is observed. Because the intensity of this peak is reduced in the composite, the other two peaks are obscured by the strong scattering from GPNFs. Both ring- and chain-like Se structures prevail in pristine Se and it’s composite. In the composite, the D band due to edge and surface defects present on the graphitic ribbons and the G band due to the vibrations of sp2-hybridized

Figure 1. SEM images of (a) pristine Se and (b,c) the Se/GPNF composite, (d) TEM image and (e) SAED pattern of pristine Se, and (f,g) TEM images and (h,i) SAED patterns of the Se/GPNF composite. GB is the grain boundary in (d).

dimensions on the order of several microns. In comparison, the Se/GPNF composite is composed of intertwined ribbon-like shapes of GPNFs, and the Se particles are distributed uniformly over these graphitic structures (Figures 1b,c). Unlike pristine Se, here, the Se particles are prevented from aggregation due to van der Waals interactions between the ribbons and the particles. The electrically conductive nanoribbons also permit facile propagation of electrons across the cross section of the cathode material during charge/discharge cycles. The TEM image of pristine Se (Figure 1d) reveals the individual grains with distinct grain boundaries but having a large variation in sizes, 100−300 nm. The corresponding selected area electron diffraction (SAED) pattern (Figure 1e) shows diffuse rings with bright spots superimposed over the rings. The spots are indexed along the (220), (113), (101), and (110) planes corresponding to d values of 0.10, 0.13, 0.29, and 0.26 nm of Se with a primitive hexagonal structure. The TEM image of the Se/GPNF composite (Figure 1f) shows uniformly distributed curved, bent, and straight ribbon-like shapes of GPNFs with Se particles. The striations observed on the ribbons (Figure 1g) are the graphitic planes parallely oriented but orthogonal to the length of the ribbons, separated by 0.34 nm. The SAED patterns extracted from the composite (Figures 1h,i) show ring patterns with a few spots. The spots and the rings are assigned to the (101), (201), and (104) planes, matching d = 0.328, 0.178, and 0.117 nm of hexagonal Se, and to the (002), (012), and (105) planes, agreeing with interplanar distances of 0.331, 0.206, and 0.114 nm of graphite. The ability of GPNFs to enhance the electrical conductivity of Se is gauged from electrical conductivity measurements. I−V characteristics of Se, GPNFs, and Se/GPNFs are recorded in a 1289

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Figure 2. (a) I−V characteristics of pristine Se, GPNFs, and Se/GPNFs; the dotted lines represent the linear fits. CV plots (first 10 cycles) of cells with (b) Se and (c) Se/GPNFs as cathodes, recorded at a scan rate of 0.2 mV s−1 in the voltage window of 1.65−3 V with Li metal as the counter electrode. (d) Schematic of lithiation and delithiation processes in Se.

Figure 3. Galvanostatic charge/discharge curves of Li−Se cells with the following cathodes: (a) pristine Se and (b) Se/GPNF composite. Cycling stability of Li−Se cells with a Se/GPNF composite cathode at rates of (c) 0.5 and 1 C and (d) 0.1 C (Se versus Se/GPNFs); the inset of (d) shows the cycling stability of Se/GPNFs for 350 cycles. (e) Coulombic efficiency comparison of Li−Se cells with pristine Se and Se/ GPNF composite cathodes at a 0.1 C rate. (f) Rate capability of Li−Se cells with pristine Se and the Se/GPNF composite recorded at different C rates.

SS/active material/SS configuration from −1 to +1 V, where the two SS plates are separated by an adhesive tape of a known thickness, which serves as a spacer. The plots are shown in Figure 2a. The behavior is almost linear for the active materials, albeit with some slight deviation from the quasi-Ohmic dependence at higher potentials, especially for GPNFs. From the straight line fits and using the slopes and the relations slope = I/V = 1/R and κ (S cm−1) = 1/R(l/a), where l is the spacer thickness, and a is the active electrode area, the electrical conductivities (κ) are calculated. κ values are 0.04, 0.004, and 0.03 S cm−1 for GPNFs, Se, and Se/GPNFs, respectively. GPNFs impart a 7-fold increment to the conductivity of Se in

the composite, and the benefit of the enhanced conductivity is reflected in the electrochemical performance of the composite. Composites of Se are also attempted with other carbon materials such as Se/GO, Se/RGO, and Se/MWCNTs, and their electrochemical performances are compared with that obtained for the Se/GPNF composite. The underlying objective here is to show that GPNFs are more effective as the conductive and buffering support for Se in contrast to GO, RGO, and MWCNTs. To study the effect of incorporating a given carbon material in the Li−Se cell, electrical conductivities of pristine carbon materials (GO, RGO, and MWCNTs) are compared with that of GPNFs (Figure S2, Supporting Information). Using the straight line fits in the I−V plots, κ 1290

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Figure 4. Simultaneous topography and current maps of (a,a′) uncycled Se and (b,b′) Se/GPNFs and (c,c′) Se and (d,d′) Se/GPNFs after the first discharge.

values are 0.0031, 0.0069, 0.0032, and 0.04 S cm−1 for GO, RGO, MWCNTs, and GPNFs, respectively. The electrical conductivity is observed to be higher for GPNFs compared to that for the remaining three carbon materials, and this plays a significant role in imparting a higher Li+ storage capacity to Se/ GPNFs in comparison to the other three composites. The redox processes of Li−Se and Li−Se/GPNFs cells are studied by cyclic voltammetry (CV). Figure 2b,c shows the initial 10 CV plots of the cells measured at a scan rate of 0.2 mV s−1 between 1.65 to 3 V versus Li+/Li. CV curves of the pristine Se and Se/GPNF composite cathodes show one cathodic peak and two anodic peaks. The cathodic peak is attributed to the reduction of ring- and chain-structured Se8 to soluble polyselenides Li2Sen (n ≥ 4), followed by reduction to Li2Se, whereas the anodic peaks correspond to sequential oxidation of Li2Se to polyselenides (Li2Sen, n ≥ 4) and then to Se.18 In the cathodic branch, in the first discharge, the Se/ GPNF composite shows a strong peak at 2.1 V and a set of anodic peaks at 2.20 and 2.27 V during the charging process. In the subsequent cycles, the anodic peaks shift to slightly higher voltages of 2.22 and 2.29 V, and the cathodic peak at 2.11 V becomes weaker and is replaced by a new peak at 2.21 V. Pristine Se exhibits one cathodic peak at 2.07 V and two anodic peaks at 2.21 and 2.3 V, and with cycling, the latter peaks shift to 2.26 and 2.33 V and the cathodic peak shifts from 2.07 to 2.11 V. During the first lithiation process, electrochemical activation occurs as ring-like Se is converted to chain-like Se and then lithiated, as shown in the schematic in Figure 2d. The peak current densities of oxidation and reduction peaks of the Se/GPNF composite are higher than that of pristine Se, indicating that the Se/GPNF composite delivers a larger capacity than pristine Se. The anodic and cathodic peaks of the Se/GPNFs composite cathode are stable after the third cycle compared to pristine Se, which does not show a stabilized response with cycling. The GPNFs in the Se/GPNF composite accommodate the structural strain and volume expansion that Se experiences during lithiation and the volume contraction during the delithiation process.

Galvanostatic charge/discharge curves of pristine Se and Se/ GPNF composite cathodes measured at the rate of 0.1 C in the voltage range of 1.65−3 V are shown in Figure 3a,b. Both charge and discharge voltage plateaus are in good agreement with the corresponding CV profiles. The potential separation between the charge/discharge voltage plateaus for the Se/ GPNF composite cathode is much smaller than that of the pristine Se cathode, thus affirming superior kinetic characteristics and reversibility of the cell with the Se/GPNF composite. The Se/GPNF composite exhibits complete and stable discharge/charge voltage plateaus at 2.19, 1.96, and 2.25 V even after 200 cycles, which implies good electrochemical stability and reversibility of the Se/GPNFs composite cathode. The cycling performances of the Se/GPNFs-based cell at higher current rates of 0.5 and 1 C are shown in Figure 3c. Capacity values are calculated on the basis of the amount of Se present in the cathode material. The Se/GPNF composite delivers initial discharge capacities of 550 mAh gSe−1 at 0.5 C and 360 mAh gSe−1 at 1 C rates and retains capacities of 159 and 69 mAh gSe−1 after 100 cycles, respectively. Figure 3d shows the comparison of cycling stability of the two cathodes at a 0.1 C rate. The initial discharge capacities of the Se/GPNF composite with 75 wt % Se and pristine Se are 847.59 and 291.33 mAh gSe−1, and these decrease to 489 and 153 mAh gSe−1 after 200 and 100 cycles with capacity retentions of 57.7 and 52.5%, respectively. Both the initial discharge capacity and durability of the composite are obviously superior to those of pristine Se. The initial capacity is greater than the theoretical capacity of Se (678 mAh g−1) for Se/GPNFs due to solid electrolyte interphase (SEI) formation and decomposition of the electrolyte on the surface of Se/GPNFs.19,20 While these phenomena can occur on the surface of pristine Se as well, from the values, it appears as if GPNFs are responsible for this enhanced discharge capacity. The first charge capacities are unusually high; they are 940 and 874 mAh gSe−1 for Se/GPNFs and Se. The initial Coulombic efficiencies of pristine Se and the Se/GPNF composite are 33.3 and 90.1%, respectively (Figure 3e). The charge capacities are 3 and 1.1 1291

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expected to be facile. The advantage of GPNFs is thus clearly reflected in the current maps. The electrode topographies are not particularly insightful after the first discharge (Figure 4c,d). However, the current maps are strikingly different upon going from Se to Se/GPNFs. While the Se/GPNF composite (Figure 4d′) retains high maximum and average currents after the first discharge and the high current domains continue to dominate the surface, the Se surface (Figure 4c′) is characterized by low currents, and the high-current regions are small and sparsely distributed. The maximum current is also very low for Se (∼0.6 nA), which is suggestive of active material loss. These differences indicate the role of GPNFs in preserving the current-carrying capability of the active electrode, even after reaction with Li-ions. To analyze the role of GPNFs, in particular, in improving the Li+ storage capability of Se, the charge/discharge characteristics of Li−Se cells with composites of Se/GO, Se/RGO, and Se/ MWCNTs as cathodes (at 0.1 C rate) in the voltage range of 1.65−3 V versus Li/Li+ are studied and shown in Figure S4 and Table S1 (Supporting Information). The initial discharge capacities of Se/GO, Se/RGO, and Se/MWCNTs composites are 350, 644, and 1122 mAh gSe−1, and these composites retain capacities of 249, 322.8, and 291.3 mAh gSe−1 at the end of 100 charge/discharge cycles, respectively. Although the first discharge capacity of Se/MWCNTs is greater than that of the Se/GPNF composite, the cycling performance is poor for Se/ MWCNTs. After 100 cycles, the Se/MWCNT and Se/GPNF composites retain ∼26 and ∼66% of their original capacities. Similarly, while the capacity retention offered by Se/GO is high (71%), the absolute magnitude of capacity retained after 100 cycles is very low, which again renders it to be not as effective as GPNFs. It is obvious that, compared to other conventional nanostructured carbonaceous materials such as GO, RGO, and MWCNTs, the Se/GPNF composite exhibits optimal balance between cycling stability and capacity, which is of paramount significance especially in view of practical applications. The high conductivity and the fibrous nature of GPNFs assist in maintaining good electrical interconnectivity between the Se particles and prevent their aggregation, thus maximizing Li+ uptake and reaction with the active Se species, which leads to a higher capacity. Dissolution of polyselenides is also minimized by the GPNFs via van der Waals interactions, and their binding with the soluble polyselenides (Li2Sen, n = 4−8) formed during discharge is strong enough to prevent their leaching into the electrolyte and, at the same time, weak enough to release the lithium ions during charge. RGO, GO, and MWCNTs tend to aggregate upon repetitive cycling, which makes them less effective in improving the Li+ uptake capacity of Se. Pristine Se exists in the form of large aggregated particles that limit the number of available active Se sites for electrochemical reaction with Li+ ions, thus reducing the capacity, despite a high loading of Se by weight (80 wt %). In the composite, GPNFs also act as a conducting additive and facilitate electron transport across the bulk of the material during lithiation and delithiation. Electron travel across the cross section of the electroactive material is as significant as Li+ ion transport and reaction with active Se. Pristine Se and the other Se/C composites are less conducting than the Se/GPNF composite, and therefore, electron transport is hindered by their lower conductivities, thus adversely affecting their Li+ storage capacity. Furthermore, in comparison to literature values of capacities for Se composites, we find that the reversible capacity performance of Se/GPNFs is better than that of most of the

times greater than the corresponding discharge capacities for Se and Se/GPNFs, respectively. After the first cycle, the SEI formation occurs on the surface of the Se/GPNF composite or Se, and therefore, the initial efficiency is low, but after some cycles, the SEI is stabilized, which is reflected in the increase in the Coulombic efficiency after some cycles. However, in case of pristine Se, initially the efficiency is low as compared to the Se/ GPNF composite due to severe active material loss after the first discharge. Furthermore, the structural distortion (volume expansion, during discharge) caused by Li-ion ingress and then volume contraction during charge by Li-ion egress in Se are much greater than what is induced in Se/GPNFs, and as a consequence, the irreversible capacity loss is much higher for Se. The Coulombic efficiency increases from 90.1 to 98% for the Se/GPNF composite cathode with cycling (after 100 cycles), and for pristine Se, the Coulombic efficiency increases from 33.3% to 97% after 100 cycles. The high Coulombic efficiency of the Se/GPNF composite indicates that (i) shuttle and active material loss effects are restrained and (ii) the structural changes are better buffered in the composite compared to that for pristine Se due to the presence of GPNFs. To understand this better, the SEM images of Se- and Se/ GPNF-based electrodes after the first discharge are compared (Figure S3, Supporting Information). The morphology of Se undergoes a noticeable change for after the first discharge, the aggregated particles of Se are not observed and an interconnected network of particles is observed. The latter is due to the formation of SEI. Large voids, few microns in diameter, are also seen, and they are probably formed by the loss of Se via dissolution of polyselenides in the electrolyte. The volume expansion that Se experiences upon Li2Se formation is therefore not easily accommodated by pristine Se. For Se/ GPNFs, after the first discharge, continuous SEI layer formation is observed without large voids, indicating that active material (Se) loss is restricted effectively by the GPNFs, and it also suggests that the volume change incurred due to Li2Se formation is accommodated by the GPNFs. The quasimesh-like framework of GPNFs accommodates the volume increase experienced by Se. The ability of Se/GPNFs to better accommodate the volume change experienced upon discharge compared to Se is also investigated by using conducting atomic force microscopy (CAFM) studies. Figure 4 shows the topography and current images recorded simultaneously for equivalent areas of asfabricated electrodes of Se and Se/GPNFs and the same after the first discharge. The topography of the uncycled Se electrode shows aggregated Se particles (Figure 4a), and for Se/GPNFs (Figure 4b), some elongated particles are observed, indicating the presence of GPNFs. In the corresponding current images (Figure 4a′,b′), the bright regions are assigned to the highcurrent-flowing domains, and the dark portions represent the low-current-carrying regions. The currents are color-scaled on the right side of the images. The maximum current for the uncycled Se/GPNFs is 5-fold times greater than that of Se. The average current is about 15 nA for the composite and 1.3 nA for Se. Besides the current magnitude differences, the high current (bright) domains are better delocalized over the Se/GPNFs electrode surface compared to the Se surface. The low-current domains have a very large surface coverage in the case of Se. It is obvious that Se/GPNF has large well-interlinked highly conducting domains, which can facilitate electron propagation during charge/discharge. Pristine Se has large relatively insulating regions, and therefore, electron movement is not 1292

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Figure 5. Nyquist plots of Li−Se and Li−Se/GPNFs cells: (a) before cycling and (b) after the 200th cycle at the rate of 0.5 C. The inset of (a) is an enlarged view of the Li−Se cell’s response. Equivalent circuits for (c) Se and (d) Se/GPNFs.

Table 1. EIS parameters for Li−Se and Li−Se/GPNFs cells before cycling

after 200 cycles 1/2

cathode

Rb (Ω)

Rct (Ω)

Rgb (Ω)

Yo (S s )

Rb (Ω)

Rct (Ω)

Rgb (Ω)

Yo (S s1/2)

Se Se/GPNFs

4.4 4.6

18.4 6.94

79.2

0.016 0.035

4.66 4.65

11.1 9.6

28.3

0.019 0.018

previous reports, except the one by Li et al.5 Briefly, for a Se/ CNTs composite,6 an initial capacity of 400 mAh gSe−1 decreased to 315 mAh gSe−1 after 100 cycles at a 0.1 C rate. For Se/C bubbles,12 the capacity dropped from 691 to 605 mAh gSe−1 at the end of 100 cycles at the same C rate. A Se/ CMK3 (porous C) composite delivered an initial capacity of 900 mAh gSe−1, which declined to 600 mAh gSe−1 after 50 cycles.15 A Li−Se cell with a carbon interlayer gave an initial value of 656 mAh gSe−1, and it reduced to 520 mAh gSe−1 after 20 cycles.10 In another study, a reversible capacity of 430 mAh gSe−1 was retained after 250 cycles at 100 mA g−1.21 In a study of note,22 a three-dimensional hierarchical graphene−carbon nanotubes@Se (3DG-CNT@Se) composite exhibited an initial capacity of 632.7 mAh gSe−1 and maintained a capacity of 504.3 mAh gSe−1 after 150 cycles at a 0.2 C rate. For Se/GPNF here, a capacity of 509.8 mAh gSe−1 is achieved after 150 cycles, and a 384.7 mAh gSe−1 capacity is retained after 350 cycles (at 0.1 C rate). Here, we have cycled the cell for 200 more cycles. The weight percentage of Se in the Se/GPNF composite is higher, that is, 75 wt % compared to the 3DG-CNT@Se, where it is 51 wt %. The method used for preparation of the composite22 is relatively more complex as it involves a solvothermal route, multiple washing steps, and freeze-drying as well, which are difficult to scale up. In comparison, here we used a direct dry mixing and grinding method of the two components (Se and GPNFs), which is facile and does not require any sophisticated equipment or elevated temperature or any inert conditions. The morphologies of the carbonaceous nanostructures are different; GPNFs are composed of interconnected ribbon-like structures of semi-graphitic carbon, whereas the 3DG-CNTs is composed of mingling CNTs and graphene sheets. There are very few reports where researchers have operated Li−Se cells for more than 100 cycles, particularly at low C rates, which clearly brings out the significance of the present study. Rate capability comparison of pristine Se and Se/GPNF composite cathodes are shown in Figure 3f. Besides cycling

stability, the Se/GPNF composite shows good rate capability. As the C rate is progressively increased from 0.1 to 0.2, 0.3, 0.5, and 1 C, the Se/GPNF composite cathode delivers discharge capacities of 632, 374, 272, 194, and 83 mAh gSe−1, respectively, and the composite recovers satisfactory discharge capacities of 171, 235, 315, and 418 mAh gSe−1 when the current rate is gradually switched back to 0.5, 0.3, 0.2, and 0.1 C rates. Pristine Se delivers discharge capacities varying from 327.5 to 42 mAh gSe−1 as the current rate increases from 0.1 to 1 C, and this cell also regains a capacity of only 122 mAh gSe−1 when the current rate returns to 0.1 C. The rate performance of the Se/GPNF composite is attributed to the improved electrical conductivity of the Se/GPNF composite cathode compared to Se. The enhanced electrical conductivity of the composite allows faster electron propagation through the cross section of the composite and rapid reaction with Li-ions, especially at high C rates. The better retention of capacity upon going back and forth between different C rates observed for the composite relative to Se is also due to the GPNFs. To further analyze the influence of GPNFs on the electrochemical performance of Li−Se batteries, electrochemical impedance spectra (EIS) and the corresponding parameters are compared for the two electrodes in Figure 5 and Table 1, prior to the first discharge and after 200 charge/ discharge cycles at a 0.5 C rate. Before cycling, the Nyquist plot for the Li−Se/GPNFs cell is composed of one depressed semicircle in the high-frequency region, which is assigned to the charge transfer resistance (Rct) operative at the Se/GPNFs/ electrolyte (LiTFSI−solvent) interface. The inclined line in the low-frequency region is attributed to diffusion of Li+ ions through the active electrode material. For pristine Se, besides these two components, an additional distorted semicircle (in the intermediate frequency (1200 Hz) to low-frequency range (1.26 Hz)) is sandwiched between the high-frequency arc and the low-frequency Warburg line. It is ascribed to the grain boundary resistance (Rgb), the resistance offered by the Se grain 1293

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boundaries in bulk Se to both electron and Li-ion transfer. Because these quasi-insulating grain boundaries are not present in the composite, this component is not observed. The equivalent circuits for Se and Se/GPNFs are shown in Figure 5c,d. The shapes of the plots do not alter with cycling for both cathodes, but the magnitudes of the associated impedances change with cycling. For uncycled electrodes, Rct is 18.4 Ω for pristine Se and 6.94 Ω for the the Se/GPNF composite. Post cycling, the values are 11.1 and 9.6 Ω, respectively. Rct is lower for the composite relative to that for pristine Se, both before and after cycling. The high electrical conductivity of GPNFs allows fast electrochemical reaction of Se with Li-ions and electrons at the electrode/electrolyte interface. The increasing thickness of electrically insulating the SEI formed at the electrode/electrolyte interface with cycling increases Rct in the composite. While the loss of active material is contained by GPNFs in the composite, it is severe in the case of pristine Se, and therefore, a decrease in active material thickness outweighs the cycling-induced increasing SEI thickness disadvantage and reduces Rct. The point of commencement of the first skewed arc is the bulk resistance (Rb) of the electrolyte, and it is found to be in the range of 3.5−4.7 Ω. In pristine Se, Rgb, like Rct, also decreases with cycling, from 79.2 to 28.3 Ω. In the lowfrequency domain, Yo represents the measure of diffusional conductance, the ease of diffusion of Li-ions through the cross section of the electroactive material. Yo is 0.035 S s1/2 for the Se/GPNF composite, which is more than 2-fold times that of Se (0.016 S s1/2). It is evident that GPNFs allow facile transport of Li-ions and electrons throughout the composite, which is also reflected in the high capacity achieved for the composite at a fairly low C rate of 0.1. In summary, a Se/GPNF composite with a high Se loading (75% by weight) is prepared by an extremely simple route, and when used as a cathode in Li−Se cells, it outperforms a pristine Se-based cell and even composites with other carbon materials. Compared to the pristine Se electrode, which is composed of large chunks of Se, the Se/GPNF composite is made up of conductive nanoribbons of graphitic structures that interlink the Se particles and inhibit their agglomeration, and the porous morphology permits efficient electrolyte penetration through the bulk of the active material, thus enabling facile reaction of Se with Li+ ions and electrons and high Se utilization during discharge. The framework of interconnected nanoribbons of GPNFs ensconces the polysulfides and prevents them from dissolving in the electrolyte and their shuttle during the repetitive charge/discharge cycles. Abetted by these aspects, the Se/GPNF composite delivers an initial capacity of 847.5 mAh gSe−1 at a 0.1 C rate, and at the end of 200 and 350 cycles, capacities of 489 and 384.7 mAh gSe−1 are retained, which are superior to (a) the reversible capacities reported previously for Se composites in bulk of the reports on Li−Se cells, (b) the pristine Se-based Li−Se cell (prepared herein, 153 mAh gSe−1, after cycling), and (c) Se/GO, Se/RGO, and Se/MWCNTs. CAFM studies also confirm that the Se/GPNF electrode surface is characterized by highly conducting domains, which facilitate electron propagation during charge/discharge. The good rate response, low polarization, and reasonable capacity fade with cycling induced by the presence of GPNFs in the cathode are performance metrics that clearly showcase the promise that the Se/GPNF composite cathode holds for scale-up and development of practical Li−Se batteries.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00251. Experimental details, XRD patterns, and Raman spectra of electroactive materials, I−V characteristics of carbon materials, SEM images of cycled electrodes, charge/ discharge curves and cyclic stability of Se/carbon material composites, and a table summarizing the electrochemical properties of Se/carbon material composites (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-4023016003. Tel: +914023016024. ORCID

Melepurath Deepa: 0000-0001-7070-5100 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Department of Science and Technology (DST) - Science and Engineering Research Board (SERB) (EMR/2015/001775) is gratefully acknowledged. R.M. thanks University Grants of Commission (UGC) of India for a senior research fellowship.

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DOI: 10.1021/acsenergylett.7b00251 ACS Energy Lett. 2017, 2, 1288−1295