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Designed Formulation of Se-Impregnated N‑Containing Hollow Core Mesoporous Shell Carbon Spheres: Multifunctional Potential Cathode for Li−Se and Na−Se Batteries Balakumar Kalimuthu†,‡ and Kalaiselvi Nallathamby*,†,‡ †

Electrochemical Power Systems Division, CSIR- Central Electrochemical Research Institute, Karaikudi 630 003, India Academy of Scientific and Innovative Research, Chennai 600 113, India

‡

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S Supporting Information *

ABSTRACT: Nitrogen-containing carbon spheres with hollow core and mesoporous shell (NHCS), capable of confining Se at levels as high as 72 wt % has been demonstrated to exhibit appreciable electrochemical behavior with 52 and 61 wt % Se loading. In particular, 52 wt % Se confined NHCS cathode exhibits 265 mAh/g at 10C rate and retains 75% of initial capacity at 2C rate up to 10 000 cycles with an insignificant decay of 0.0025% per cycle, which is an ever first report on the extended cycle life of Li−Se batteries. Due to the negligible difference found between the transport kinetics of Se and that of Li2Se, irrespective of the cycling rate, 52 wt % Se @ NHCS performs better at high rates. Furthermore, capacity is governed by the extent of utilization of confined Se and cycle life by the extent of mitigation of volume expansion. Accordingly, rate capability studies recommend 52 wt % Se loaded cathode above 2C rate and 61 wt % Se loading up to 2C rate. Furthermore, NHCS/Se-52 cathode demonstrates suitability for Na−Se batteries by exhibiting 339 and 219 mAh/g of capacity at rates of C/5 and 2C rates, respectively. NHCS with select Se concentration could thus be exploited for multifunctional cathode behavior in Li−Se and Na− Se systems. KEYWORDS: N-doped hollow carbon spheres, mesoporous nanoshell carbon, cyclability, Li−Se batteries, Na−Se batteries

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INTRODUCTION Limited availability of nonrenewable energy resources and threatening environmental pollution issues prompt the need for the development of newer and renewable energy storage devices with appreciable cycle life and capacity to address the requirements of electric and hybrid electric vehicles (EVs and HEVs). In this regard, Li-ion batteries are the proven candidates with market winning performance strategy to cater to the energy requirements of existing EVs and HEVs to a greater extent, but its limitations related to theoretical capacity and energy density pose severe restriction against extensive usage.1−4 Alternatively, lithium-chalcogen batteries such as Li− S, Li−Se, and Li−Te systems not only have higher theoretical capacity and energy density but also exhibit a vast scope of practical viability and application by proper electrode design.5−7 In particular, Li−S fulfills the demand for high-capacity energy storage systems in a major way.8−11 Next to Li−S, lithium− selenium (Li−Se) is considered as the alternative and potential energy storage system for EV and HEV applications due to its higher volumetric capacity (3253 mAh/cm3), comparable with that of Li−S system (3467 mAh/cm3). Furthermore, Se possesses inherent advantages such as electronic conductivity on the order of 10−5 S/cm (as a d-electron containing element) better than that of its congener S (∼10−30 S/cm) and weaker shuttle effect, which are in favor of the high rate performance © 2017 American Chemical Society

and maximum utilization of electrochemically active Se material.12 On the basis of these reasons, Li−Se system has been chosen for the current study, and the scarcely available reports on Li−Se batteries trigger the need to gain more understanding about the system, especially in terms of development of potential cathode materials, validation, and recommendation of promising candidates along with the provision of more insights on the mechanism involved and factors governing the performance of electrode. It is this connection that the current study assumes importance in providing detailed information about the synthesis−structure− property relationship that plays a pivotal role in identifying potential active material confining cathode candidate for Li−Se system. Similarly, yet another feasible and futuristic energy storage strategy based on Na−Se batteries has also been explored with a carefully chosen optimized cathode formulation and demonstrated the multifunctional capability of NHCS-Se cathode through the present study. Basically, electrochemical reversible storage of Li by Se can be described as Se + 2Li+ + 2e− ↔ Li2Se,12 corresponding to a theoretical capacity of 675 mAh/g, due to multielectron Received: April 13, 2017 Accepted: July 18, 2017 Published: July 18, 2017 26756

DOI: 10.1021/acsami.7b05103 ACS Appl. Mater. Interfaces 2017, 9, 26756−26770

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Scheme of Steps Involved in the Synthesis of Se-Impregnated N-Containing Hollow Core Mesoporous Shell Carbon Spheresa

a

NHCS/Se-X, where X indicates the amount of Se.

due to N doping creates a polar nature that facilitates the favorable electrolyte access.25 Similarly, strong affinity of Se and Li2Se with that of N-doped carbon prevents the loss of active material from the conductive network, which in turn is beneficial for the better confinement of active material and charge transfer kinetics, required to demonstrate superior electrochemical properties of Se cathode.26,27 Above all, Ndoping provides facile electronic conductivity even at high rates and extended cycling. Quite different from previous reports,19,21,28 the current study discusses on the structural, morphological and electrochemical advantages offered by using NHCS/Se cathode. Higher Se confinement is achieved in its amorphous form, but a lesser concentration of crystalline Se was found to be intact with the NHCS. Individual and perfectly formed N-containing hollow core mesoporous shell carbon without any undesirable agglomeration and the resultant cathode with comparatively lesser polarization behavior have been demonstrated. Nitrogen-containing hollow carbon spheres (NHCS) with mesoporous shell have been synthesized using silica hardtemplate method, wherein pyrrole serves as a heteroatom (N)containing carbon precursor29,30 because structurally stable and thin shell of NHCS can provide continuous and faster Li-ion access to the confined Se by maximizing the electrolyte contact area and facilitates faster electron transport. In contrast, the hollow structure may mitigate the mechanical stress of the electrode that arises due to the repetition of huge volume changes of active material upon cycling. On the basis of these structural advantages, NHCS are believed to increase the actual utilization of active material. Interestingly, NHCS of the present study admit Se loading up to 72 wt % and exhibit superior electrochemical performance with respect to 52 and 61 wt % Se loading, due to the trade-off between filled and unfilled pores in cushioning the volume expansion to the desired level. In particular, the nanoregime shell thickness of the porous hollow carbon sphere improves the reaction kinetics of 52 wt % Se @ NHCS cathode to the extent that 75% of the initial capacity is maintained up to 10 000 cycles at 2C rate, which is noteworthy. As an extended application, the optimized composition of 52 wt % of Se @ NHCS also has been explored for its possible cathode behavior in Na−Se batteries. A nominal capacity of ∼340 mAh/g at C/5 rate has been observed in Na−Se system apart from its tolerance up to 2C rate. Hence, the study deals with the demonstration of multifunctional capability of NHCS loaded with Se to perform

transfer. Even though the theoretical capacity of Se is lower than that of S (1675 mAh/g), the higher density of Se (nearly 2.4 times higher than S with 2.07 cm3/g) offsets this disadvantage by offering comparable volumetric capacity.12 However, Se also suffers from limitations such as quick capacity fade due to factors such as loss of active material by the dissolution of polyselenide intermediates and the loss of integrity between active material and conductive host/electrode due to volume changes associated with repeated lithiation and delithiation processes. Such issues become severe, especially when bulk/crystalline Se is employed as electrode,12−14 wherein the volume expansion is about 180% (based on the densities of Se (4.81 g/cm3) and Li2Se (2.00 g/cm3)). In an attempt to mitigate such problems, dispersion of Se over MWCNT has been considered, which is reported to end up with rapid fade in capacity and increased polarization upon cycling.12 Similarly, deployment of porous bulk Se and conductive coating on bulk Se only shows less improvement.13,14 In order to overcome these hurdles, porous carbon matrices with meso-/microporous nature and candidates such as graphene and hybrid structure showing excellent electrochemical performance have been investigated.15−23 Electrochemical reaction that takes place via single-step solid−solid transformation of Se to Li2Se during discharge and vice versa on charge imposes problems related to sluggish Li+ diffusion kinetics.12 Hence, rational design of porous carbon assumes importance, wherein higher surface area, beneficial for electron transport and higher pore volume for uniform distribution and confinement of Se, along with an increase in electronic conductivity are some factors to be considered with due credit. However, with a view to combat the volume expansion of Se upon extended cycles,12,19 one can manipulate and adjust to maintain the ratio of filled and unfilled pores of the carbon matrix in such a manner that the undesirable volume expansion is mitigated either partially or predominantly, i.e., a certain percentage of free volume in carbon matrix is deliberately essential as a buffer to minimize the consequences of volume expansion. In addition, it is well-known that heteroatom containing carbon is bestowed with the conjugation, existing between the lone pair of electrons of the heteroatom and the π system of carbon that offers beneficial electronic conductivity. Furthermore, the smaller size of N atom and the creation of nanopores in carbon lattices increases the lithium ion transfer kinetics of Se cathode.24−26 Moreover, variation in the local charge densities of the carbon network 26757


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Figure 1. (a) XRD patterns of bulk Se, NHCS, and NHCS/Se-X (X = 52, 61, and 72) composites, (b) N2 adsorption/desorption isotherm, and (c) pore size distribution of NHCS and NHCS/Se-52 and NHCS/Se-61 composites.

NHCS/Se-61 composites. NHCS and NHCS/Se-52 composite exhibit type-IV isotherm with H2 hysteresis loop, which corresponds to the mesoporous nature of the NHCS material.32 The isotherm of NHCS/Se-52 and NHCS/Se-61 composites exhibit a decrease in adsorbed volume with the increasing selenium content, which is an indication that Se occupies the mesopores of NHCS. Similarly, pristine NHCS possesses a specific surface area of 1000 m2/g, which is found to get reduced to 135 and 39 m2/g for NHCS/Se-52 and NHCS/Se61 composites, respectively. Pore size distribution curve of NHCS (Figure 1c) confirms the mesoporous nature of the carbon shell. Similarly, the maximum population of pores is located at 3.75 nm with a total specific pore volume of 1.04 cm3/g. NHCS retains a pore volume of 0.32 and 0.10 cm3/g after 52 and 61 wt % Se impregnation. Such a decrease in pore size distribution and pore volume of NHCS/Se-52 and NHCS/ Se-61 composites confirms the successful impregnation of Se in the mesoporous NHCS. Particularly, retention of 0.32 cm3/g pore volume of NHCS after 52 wt % Se impregnation is expected to be beneficial to tolerate the volume changes associated with the active material during cycling, based on the aforesaid trade-off factor related to filled/unfilled pores vs volume change. Raman Behavior. Raman analysis (Figure S2) displays the disordered and graphitic nature of carbon present in NHCS/ Se-X composites. D (A1g mode) and G bands (E2g mode) appear around 1349 and 1576 cm−1, respectively, and the ID/IG ratio, used to evaluate the degree of graphitization of carbon, is found to be 1.00, corresponding to the presence of disordered/ amorphous carbon in the as-prepared NHCS.19 Herein, Se impregnated at different concentrations in the NHCS composites shows no significant deviation of ID/IG value from that of pristine NHCS (ID/IG values of 1.03, 0.99, and 1.03 corresponding to NHCS/Se-52, NHCS/Se-61, and NHCS/Se72), which is an indication that Se loading does not alter the degree of graphitization of NHCS, as derived already from XRD results. Absence of characteristic Se peak in Raman spectrum confirms the confinement of Se in the pores of NHCS.19 Electrical conductivity of NHCS and NHCS/Se-52, NHCS/Se-61, and NHCS/Se-72 composites was measured using four probe method, which is about 385, 194, 17, and 2 mS/cm, respectively. This is an indication that the currently prepared NHCS that acts as a scaffold to host the semiconducting Se improves the conductivity of the NHCS/Se-X composites significantly. Morphology Studies. Morphological features of the sacrificial silica template, NHCS and a typical composite, viz. NHCS/Se-52 composite were analyzed using FESEM/

as potential cathode material in rechargeable lithium and sodium batteries involving selenium.

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RESULTS AND DISCUSSION Scheme 1 illustrates the various steps involved in the synthesis of mesoporous hollow carbon nanospheres. Colloidal silica in the form of solid spheres, also called Stöber silica spheres, were synthesized by the hydrolysis and cross-linked by TEOS in ammonia containing water−ethanol solvent. Subsequent cocondensation of TEOS and C18-TMS porogen with the asformed colloidal silica results in the formation of silica core− shell framework. The C18-TMS porogen acts as a uniform pore size directing agent by forming a covalent bond with the silica framework during the shell forming step.29−31 Upon drying, silica beads are formed. Porogens in the beads were eliminated by calcination at 550 °C in air. Removal of organic moiety in silica beads leaves the well ordered and uniform mesopores in the shell. Porous shell of silica beads holds the FeCl3 catalyst and polymerizes pyrrole, which is the N-containing carbon source.31 Carbonization of polypyrrole-containing silica beads and removal of sacrificial silica beads using HF results in the formation of N-doped hollow carbon sphere with mesoporous shell. Finally, selenium loading was done by melt diffusion strategy. Physical Characterization. Phase Analysis. Figure 1a compares the powder X-ray diffraction pattern of bulk selenium (Se) used for impregnation and the currently prepared NHCS along with the series of NHCS/Se-X products. Appearance of broad peaks centered at 2θ = 25.4 and 44° corresponding to (002) and (101) planes of graphite indicates the amorphous nature of currently prepared NHCS. Diffraction peaks of bulk Se match with the pattern of hexagonal Se (Powder Diffraction File 06−0362, International Centre for Diffraction Data, 2003). Absence of characteristic crystalline Se peaks or the presence of broad amorphous peaks found with the NHCS/Se-X composites indicates the presence of melt diffused Se into the pores of NHCS. No trace of crystalline Se is found up to 72 wt % loading, which is evidence for the highly porous structure of currently prepared NHCS and its ability to confine Se up to 72 wt %. Thermogravimetry Analysis. Thermogravimetry analysis of NHCS and NHCS/Se-X (X = 52, 61, and 72) composites was carried out under N2 atmosphere with a heating ramp rate of 10 °C/min, and the results are shown in Figure S1. From weight loss calculations, the amount of impregnated Se is found to be about 52, 61, and 72% respectively. Surface Area Analysis. Figure 1b shows the N2 adsorption and desorption isotherm of NHCS and NHCS/Se-52 and 26758


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Figure 2. (a) FESEM image, (b−d) HRTEM images of NHCS and (e−k) 52 wt % Se loaded NHCS composite, illustrating the presence of hollow core and mesoporous shell nature of NHCS (inset of (d) and (k): SAED pattern of NHCS and NHCS/Se-52 composite). (l) STEM image, (m−o) individual and (p) cumulative elemental mapping of NHCS/Se-52 electrode.

HRTEM images of NHCS/Se-52 composite captured under lower magnification (Figure 2e−g) evidence the indistinguishable difference found with that of pristine NHCS. Figure 2h,i evidence the presence of Se in the shell. Hence, it is understood that Se is not impregnated into the hollow core. In addition, higher magnification images of the shell (Figure 2j,k) contain the characteristic pattern of fringes pertinent to carbon in NHCS/Se-52 composite. The HRTEM image captured at the edge of the shell (Figure 2k) evidences the presence of pores in NHCS. The SAED pattern of NHCS/Se-52 (inset of Figure 2k) depicts the amorphous nature of Se in NHCS, which is in accordance with the XRD result. Absence of bulk Se at higher/ lower magnification images suggest that all the impregnated Se resides within the mesopores of NHCS. Figure S5 shows the STEM image and elemental mapping of pristine NHCS, which demonstrates the uniform distribution of N in the carbon shell. Furthermore, STEM mapping of NHCS/Se-52 composite (Figure 2m−p) evidences the homogeneously distributed Se confined in the porous shell of NHCS. The significant graphitic nature of porous NHCS with Se uniformly impregnated in the pores will be beneficial in terms of the effective utilization of Se,

HRTEM and the captured images are shown in Figures S3, 2a− d, and 2e−k, respectively. Figure S3a−d shows the HRTEM images of silica beads and confirms that solid silica spheres are uniformly coated with porous silica shell. The solid core has a thickness of about 150 ± 40 nm with a shell thickness of about 40 ± 5 nm. Amorphous nature of the template is evident from the corresponding SAED pattern (Figure S3e). Figure 2a shows the FESEM image of NHCS that clearly visualizes the presence of uniformly distributed nano carbon spheres. HRTEM images presented in Figure 2b−d confirm the replica structure of silica template possessed by NHCS, by way of retaining identical core and shell thickness. Interestingly, most of the spheres are found to possess uniform size and contain hollow core structure and porous shell morphology, as revealed by Figure 2b. Furthermore, Figure 2d illustrates the presence of characteristic short-range fringes of graphite, with a d-spacing value of 0.35 nm. For more clarity, the magnified fringes are displayed in Figure S4a, and the corresponding SAED pattern (inset of Figure 2d) shows the amorphous nature of NHCS. Interestly, abundantly available pores on the shell along with the hollow core structure offers ample scope for the easy percolation of electrolyte and facilitation of facile Li+ transport. 26759


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Figure 3. Deconvoluted XPS photoelectron line of (a) C 1s, (b) O 1s, and (c) N 1s of NHCS and (d) C 1s, (e) N 1s, and (f) Se 3d of NHCS/Se-52 composite evidencing the presence of pyridinic, pyrrolic, and quaternary N atom and presence of Se in two different environment.

Table 1. Peak Assignment Details of XPS Analysis of NHCS and NHCS/Se-52 Composite NHCS peak

C 1s

N 1s

O 1s

NHCS/Se-52 composite

peak label as given in Figure 3

binding energy (eV)

area ratio

peak label as given in Figure 3

binding energy (eV)

area ratioa

assignment

C−I C−II C−III C−IV N−I N−II N−III

284.89 286.20 287.80 289.85 398.48 400.34 401.32

1.00 0.22 0.12 0.11 0.65 1.00 0.60

O−I O−II O−III

530.40 531.70 533.10

0.78 1.00 0.98

C(Se)-I C(Se)-II C(Se)-III C(Se)-IV N(Se)-I N(Se)-II N(Se)-III N(Se)-IV N(Se)-V O(Se)-I O(Se)-II O(Se)-III Se 3d5/2 Se 3d3/2 Se−I

284.90 286.20 287.80 289.85 398.48 400.34 401.32 407.21 408.58 530.44 531.70 533.10 55.27 56.34 59.30

1.00 0.21 0.12 0.10 0.41 1.00 0.70 0.24 0.31 0.81 0.97 1.00 0.59 0.39 1.00

C−C/CC C−O/C−N CO/CN O−CO pyridinic N pyrrolic N quaternary/graphitic N N−Se N−Se quinone/pyridone CO C−O 3d5/2 spin−orbital coupling 3d3/2 spin−orbital coupling N−Se, Se−C and/or Se−O

Se 3d a

Area ratio between deconvoluted peak and maximum area containing deconvoluted peak in a particular photoelectron spectrum.

a high rate tolerance of the composite cathode, and the enhanced integrity of the electrode upon extended cycling. Surface Composition Analysis. Surface composition and the elements present in NHCS and NHCS/Se-52 composite were evaluated using XPS analysis. The XPS survey spectrum of NHCS (Figure S6a) contains three peaks at 289.3, 400.7, and 531.6 eV, revealing the presence of C, N, and O respectively along with the presence of Se in NHCS/Se-52 composite. Herein, Se exhibits three characteristic peaks at 233.6, 165.8, and 59.5 eV, corresponding to the 3s, 3p, and 3d orbitals of Se. Deconvoluted spectra of NHCS and NHCS/Se-52 composite are shown in Figures 3a−f and S6b. Binding energy of peaks along with their assignment and the calculated ratio of peak area are consolidated in Table 1.19,33−35 NHCS contains highly active pyridinic (N-6) and pyrrolic (N-5) N, 29 and 45%, respectively, and the remaining 26% of N is present in the quaternary/graphitic (N-Q) environment of the carbon matrix.

Elemental analysis of NHCS reveals that N content in NHCS is 6.7%, which is closer to the amount calculated from XPS (6.3%), thus evidencing the homogeneous distribution of N functionality in carbon matrix. Oxygen content may arise from physical/chemical adsorption of oxygen/moisture from atmosphere during the synthesis and/or due to HF wash. In contrast, the N 1s spectrum shows higher binding energy peaks in addition to NHCS signatures at 407.21 (N-6(1)) and 408.58 eV (N-5(1)). Similarly, the deconvoluted Se 3d spectrum contains a higher binding energy peak in addition to the characteristic elemental Se peaks (at 55.27 and 56.34 eV corresponding to 3d3/2 and 3d5/2 spin−orbital coupling) at 59.30 eV. After Se impregnation, distribution of N-6, N-5, N-Q, N-6(1), and N-5(1) in NHCS/Se-52 composite are found (Figure 3e) to be 15, 38, 22, 9, and 12%, respectively, which is an indication that quaternary N is not affected due to the impregnated Se. In fact, the binding energies of N-IV and N-V 26760


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Figure 4. Cyclic voltammograms of (a) NHCS/Se-52 and (b) NHCS/Se-61 cathodes recorded at 0.1 mV/s sweep rate indicating the excellent reversibility behavior.

cathode. As a result, NHCS/Se-52 cathode exhibits reduction peak at 1.72 V, whereas NHCS/Se-61 cathode left with inferior lithium diffusion kinetics exhibits reduction peak only at 1.58 V. For NHCS/Se-72 cathode (Figure S8), the same reductions occur at 2.61, 2.29, and 1.25 V, respectively. In addition, striking similarity with respect to the position of anodic peak (∼2.2−2.3 V) has been noticed with NHCS/Se-61 and NHCS/Se-72 cathodes. The observed identical behavior of NHCS/Se-52 and NHCS/Se-61 cathodes upon initial oxidation and subsequent cycling indicates the involvement of similar mechanism, governing the reversibility of said cathodes. However, shifting of peak potential and the observed increase in peak current values of NHCS/Se-72 cathode as a function of increasing cycle number could be correlated to the subsequently triggered electrochemical activation process.16 This might be due to the less confinement of higher (72 wt % Se) content in the NHCS host. Charge−Discharge Studies. Galvanostatic charge−discharge cycling of pristine NHCS and the corresponding Se loaded NHCS/Se-X composites was performed in the potential window of 1.0−3.0 V. The capacity calculation was made based on the amount of Se in the composite cathode (1C rate = 675 mA/g). Figure S9 corresponds to the charge/discharge behavior of NHCS, wherein an inferior capacity of ∼40 mAh/g has been observed. Similarly, inferior Coulombic efficiency of 90% after 40 cycles. However, the data carries no great significance, as the experiment has been done with a scientific curiosity to understand the behavior of NHCS, as a preliminary study. Figure S10 displays the first discharge−charge curve of NHCS/ Se-52 and NHCS/Se-61 cathodes under C/5 rate. Initial irreversible discharge plateau that appears in the region of 2.35−2.20 V gives rise to a capacity of ∼20 mAh/g, which may be due to the conversion of cyclic Se8 into a chainlike Sen molecule, and the main reduction plateau that appears at ∼1.8 V corresponds to the formation of Li2Se.17 Subsequent charging plateau at ∼2.20 V indicates single-step solid−solid transformation of Li2Se into a chainlike Sen. Particularly, NHCS/Se-52 cathode exhibits an initial charge capacity of 515 mAh/g and Coulombic efficiency of 53%, whereas a corresponding capacity of 569 mAh/g and Coulombic efficiency of 54% have been exhibited by NHCS-61 cathode. Figure 5a shows the charge and discharge profile of NHCS/Se52 cathode, which coincides with the CV behavior. Two plateaus are observed in the discharge curves with lesser potential separation, wherein the first plateau contributes less than 100 mAh/g to the total discharge capacity of 550 mAh/g.

peaks are in the range of nitrate ester, and the formation of such nitrate ester is not observed in C 1s core level spectrum of NHCS/Se-52.36 Generally, a linear chainlike Sen molecule formed after the first cycle exhibits higher binding energy in Se 3d core level spectrum due to the interaction between chainlike Sen and that of the carbon matrix to form C−Se or O−Se bond.17 Quite different from such conventional observations, presence of an additional peak in Se 3d spectrum is found with the currently investigated NHCS/Se-52 composite along with the presence of high energy peaks in the N 1s spectrum. Such an observation infers the possible interaction between highly active pyridinic (N-6) and pyrrolic (N-5) N group with that of the chainlike Sen molecule, which is believed to be formed during the melt diffusion process itself. In addition, the higher binding energy peak of Se may be correlated with the Se−O and Se−C bonds that are formed due to the surface oxidation and/or reaction of Se with the oxygen group present in NHCS.17 Electrochemical Characterization. CV Studies. Electrochemical performance of NHCS/Se-52, NHCS/Se-61, and NHCS/Se-72 cathodes has been evaluated to realize the structural advantage of NHCS in improving the electrochemical performance of Se. Cyclic Voltammogram of pristine NHCS (Figure S7) and those of NHCS/Se-52, NHCS/Se-61, and NHCS/Se-72 cathodes were recorded in the potential window of 1.0−3.0 V with a scan rate of 0.1 mV/s (Figures 4 and S8). Figure S7 depicts the CV behavior of pristine NHCS cathode and signifies the insignificant response after first cycle, which is not unusual. The first cathodic scan of NHCS/Se-52 cathode (Figure 4a) demonstrates two reduction peaks at 2.35 and 1.72 V, corresponding to the conversion of cyclic Se8 into a chainlike (linear) Sen molecule that reduces further to form Li2Se.17 The corresponding anodic scan consists of one strong oxidation peak at 2.22 V that signifies the single-step conversion of Li2Se into a chainlike Sen molecule.15,17 Subsequent cycles show identical behavior with two reduction peaks located at 2.00 and 1.86 V, governed by the reaction of lithium with two different Se molecules possessing different chain lengths.18 All the anodic curves overlap with the first cycle curve of NHCS/Se-52 cathode, which is an indication of superior reversibility of 52 wt % Se in NHCS, but NHCS/Se-61 cathode (Figure 4b) exhibits first cycle cathodic peaks at 2.51, 2.22, and 1.58 V, corresponding to the reduction of loosely bound Se to form higher order polyselenide and the conversion of cyclic Se8 into chainlike Sen molecule along with the formation of Li2Se. Herein, due to the availablility of lesser number of pores, Li-ion diffusion path length increases for the active material in the NHCS/Se-61 cathode compared with that of NHCS/Se-52 26761


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Figure 5. Galvanostatic study of NHCS/Se-52 and NHCS/Se-61 cathodes at different current rates in the potential window of 1−3 V vs Li. (a, d) Select cycle voltage profile at C/5 rate, (b, e) polarization (potential difference between the discharge and charge capacity at half of full discharge capacity), and (c, f) cyclability under the influence of C/5, C/2, 1C, and 2C rates of NHCS/Se-52 and NHCS/Se-61 cathodes, respectively.

Identical voltage profile trend observed upon charging and discharging of NHCS/Se-52 cathode upon progressive cycles exhibits insignificant variation in the capacity up to 500 cycles, which in turn signifies the advantageous role of NHCS in ensuring reversibility of Se. Moreover, reversibility of NHCS/ Se-52 cathode has been evaluated under different current rate conditions, i.e., C/2, 1C, and 2C rates, and the corresponding voltage profile is shown in Figure S11a−c. Interestingly, NHCS/Se-52 cathode exhibits perfect overlapping of charge and discharge curves up to 1000 cycles under C/2 rate. In contrast, potential separation between charge and discharge plateaus, as an indirect measure of polarization of electrode, decreases with the increasing cycle number up to 250

cycles when NHC/Se-52 cathode was subjected to 1C and 2C rate. Beyond 250 cycles, there is no noticeable difference in polarization observed up to 1000 cycles. Such an observation indicates the structural stability and better tolerance for high rate conditions bestowed with the NHCS/Se-52 cathode, prepared through the current study. A measure of polarization at 50% DOD has been done up to 1000 cycles as a function of different current rates, and the result is displayed in Figure 5b. We note that the initial polarization that starts at 0.32 V in the first cycle is found to get reduced to 0.28 V (50th cycle) and 0.23 V (500th cycle) at the C/5 rate, which is evidence for the better confinement of Se in NHCS/Se-52 cathode. Similarly, the polarization decreases from 0.34 V (first cycle) to 0.25 V 26762


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Table 2. Discharge Capacity and Coulombic Efficiency Values of NHCS/Se-52 and NHCS/Se-61 Cathode at Different Cycle Number and under Various Rate Conditions C/5 rate cathode

NHCS/Se-52

NHCS/Se-61

a

C/2 rate

1C rate

2C rate

cycle no.

capacity (mAh/g)a

CE (%)b

capacity (mAh/g)

CE (%)

capacity (mAh/g)

CE (%)

capacity (mAh/g)

CE (%)

2 10 50 100 250 500 750 1000 2 10 50 100 250 500

545 538 527 544 527 541

90.7 98.8 99.7 99.6 99.2 98.5

602 583 580 585 560 497

96.9 98.7 99.9 100.0 99.9 99.2

493 480 484 480 469 463 453 443 557 567 557 563 557 502

100.3 100.9 101.4 99.7 99.0 100.8 101.1 100.5 100.0 99.8 99.7 99.6 99.3 98.6

443 469 463 458 475 391 416 387 515 512 501 492 436 424

99.9 100.0 98.8 99.0 99.6 101.0 100.0 98.6 99.2 98.1 100.5 100.0 99.8 100.4

410 405 411 429 445 391 364 383 469 477 450 392 285 222

98.3 99.7 99.9 99.7 100.1 99.9 99.9 100.1 96.6 99.8 100.0 99.5 99.3 100.0

Specific discharge capacity. bCoulombic efficiency.

cathode at the C/5 rate is shown in Figure 5d. Two discharge plateaus and a single charge plateau are observed from the second cycle onward, corresponding to the NHCS/Se-61 cathode, which is identical to that of the NHCS/Se-52 cathode. However, the NHCS/Se-61 cathode delivers higher discharge and charge capacity than those of the NHCS/Se-52 cathode. Figure S12a−c shows the profile of NHCS/Se-61 cathode cycled at C/2, 1C, and 2C rates. The appearance of identical plateaus with increasing polarization behavior has been observed for all the rates after 100 cycles. Figure 5e depicts the variation of polarization as a function of cycle number, corresponding to different rates. Herein, the NHCS/Se-61 cathode cycled at C/5, C/2, 1C, and 2C rates shows polarization to the extend of 0.26, 0.26, 0.32, and 0.47 V during the initial cycle, 0.21, 0.23, 0.33, and 0.48 at the 100th cycle, and 0.71, 0.42, 0.77, and 0.69 at the 500th cycle, respectively. Interestingly, it is found that the rate of cycling affects the polarization to a greater extent up to 100 cycles. For moderate rates such as C/5 and C/2, a decrease in polarization is observed, and it is almost constant and begins to increase with higher (1C and 2C) rates. Beyond 100 cycles, it shows higher polarization than that at the 100th cycle for each current rate, which could be correlated to the slightly lower conductivity of higher Se content and the poor or insignificant sequestering effect of Se against continuous volume changes anticipated during cycling. We note that the reduced polarization observed up to ∼100 cycles is responsible for the observed high capacity values of NHCS/Se-61 cathode compared with that of NHCS/ Se-52 cathode. Similarly, the increased polarization that leads to reduced capacity behavior beyond 100 cycles of NHCS/Se-61 cathode is correlated to the higher concentration of Se (than that of the preferred optimum amount), which in turn is responsible for the insufficient buffering of volume expansion occurring upon cycling. High Se loading leaves a lesser void volume in terms of unfilled pores, which are not effective in addressing the volume expansion related issues of the NHCS/ Se-61 cathode. Figure 5f summarizes the cycling behavior of NHCS/Se-61 at C/5, C/2, 1C, and 2C rate, respectively. Cathode cycled at C/5 rate delivers an initial capacity of 602 and 497 mAh/g at the end of 500 cycles with a Coulombic efficiency of 99.2%. The

(1000th cycle), when cycled under a C/2 rate, and a variation of polarization from 0.52 V (1st cycle) to 0.34 V (1000th cycle) has been noticed under the influence of a 1C rate. More interestingly, the 2C rate results in much reduced polarization behavior (0.65 V in the first cycle and 0.38 V in the 1000th cycle), corresponding to a difference of 0.27 V up to 1000 cycles. Hence, it is understood that the electrochemical activation process under high rate condition is highly favored, as far as NHCS-52 cathode is concerned, which is the highlight of 52 wt % Se loaded cathode investigated through this study Figure 5c depicts the capacity versus cycle number behavior of NHCS/Se-52 cathode studied as a function of rates such as C/ 5, C/2, 1C, and 2C. At C/5 rate, the cathode exhibits an initial capacity of 545 mAh/g with a Coulombic efficiency of 91% with a better retention of capacity up to 500 cycles. At the end of 500 cycles, a capacity as high as 541 mAh/g with an excellent Coulombic efficiency of 98.5% has been observed, thus indicating the cyclability, retention capability, and stability of NHCS/Se-52 cathode upon extended cycles. Similarly, 90% of the initial capacity is found to be retained up to 1000 cycles when cycled under C/2 rate. Furthermore, excellent retention of capacity, up to 87 and 93% up to 1000 cycles, has been observed at 1C and 2C rate conditions, indicating the effective utilization of Se upon cycling under high rate conditions. In other words, Se impregnated into the NHCS matrix offers reduced polarization, improved capacity, better retention, and excellent Coulombic efficiency under extended cycling and rate conditions. Similarly, the electrochemical performance of 61 wt % of Se loaded NHCS was evaluated under different rates of current to understand the feasibility of exploring the electrode for highenergy density applications of the Li−Se system. Figure S10 shows the first discharge−charge curve of NHCS/Se-61 cathode at a C/5 rate. As observed in CV, the first discharge curve consists of one additional irreversible plateau at 2.49 V with a capacity of 80 and 45% of the capacity retention at 1C and 2C rate conditions even after 500 cycles with a Coulombic efficiency of ∼100% for a higher Se content of 61 wt % highlights the advantage of hollow core and mesoporous carbon spheres in ensuring the appreciable performance of high energy density Li−Se system with extended cycle life behavior. Hence, the observed inferior electrochemical behavior of NHCS/Se-72 cathode (as discussed in the Supporting Information) compared with those of 52 and 61 wt % Se loaded NHCS cathodes gives a direction that further comparison of electrochemical performance may be restricted with NHCS/ Se-52 and NHCS/Se-61 cathodes for select characterizations. Rate Capability and Extended Cycling. Rate capability study of NHCS/Se-X (X = 52, 61, and 72) cathodes was performed to further understand the role of NHCS in enabling the electrochemical utilization of Se and the result is summarized in Figures 6a−d and S13b. Figure 6a,b displays the voltage profiles of NHCS/Se-52 and NHCS/Se-61 cathodes under the influence of C/5, C/2, 1C, 2C, 5C, 7C, and 10C rates. Retention of discharge−charge plateau even at a 10C rate for NHCS/Se-52 cathode and up to the 7C rate for NHCS/Se-61 cathode proves the promising advantages of NHCS that acts as a host matrix for Se cathode. Figure 6c compares the polarization behavior of NHCS/Se-52 and NHCS/Se-61 cathodes at different C rates, wherein polarization to the extent of 0.26, 0.45, and 0.93 V for NHCS/Se-52 cathode and 0.27, 0.47, and 1.42 V for NHCS/Se-61 cathode at C/5, 2C, and 10C rates have been observed. Polarization is insignificant up to 1C, and a gradually increasing polarization

behavior has been noticed for both NHCS/Se-52 and NHCS/ Se-61 cathodes with the increasing C rates, such as 2, 5, 7C, and 10C. It is evident from Figure 6d and Table 3 that the capacity of NHCS/Se-61 cathode is superior with respect to that of the 52 wt % loaded cathode up to the 2C rate, whereas the capacities at 5C, 7C, and 10C are found to be superior with respect to that of 52 wt % Se loading. This observation leads to an understanding that Se present in pores plays a vital role in deciding the rate capability behavior up to 2C and that the Table 3. Discharge Capacity Value Exhibited by NHCS/Se52 and NHCS/Se-61 Cathode during Rate Capability Study and That of NHCS/Se-52 Cathode during Extended Cycling at 2C Rate Specific Discharge Capacity (mAh/g) rate C/5 C/2 1C 2C 5C 7C 10C

26764

NHCS/Se-52 cathode

NHCS/Se-61 cathode

539 505 475 425 357 301 265 Extended Cyclability of NHCS/Se-52 Cathode at

602 576 536 476 328 227 93 2C Rate

cycle no.

specific discharge capacity (mAh/g)

Coulombic efficiency (%)

2 100 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

410 429 383 296 305 299 269 286 291 304 297 307

98.3 99.7 100.1 99.8 100.2 100.0 99.7 99.6 98.6 100.3 100.1 100.2 DOI: 10.1021/acsami.7b05103 ACS Appl. Mater. Interfaces 2017, 9, 26756−26770

Research Article


Figure 7. Electrochemical impedance behavior of cells containing NHCS/Se-52 and NHCS/Se-61 cathodes individually as a function of depth of charge and discharge under different rates. (a) Potentials selected in voltage profile for EIS measurement (B.F., EIS measurement before discharge), variation of (b) charge transfer resistance, and (c) diffusion coefficient with respect to the state of charge and discharge exhibited by NHCS/Se-52 and NHCS/Se-61 cathodes at select rates.

cycles with better Coulombic efficiency and an insignificant capacity decay of 0.0025% per cycle, which in turn substantiates the excellent reversibility of duly confined Se in NHCS/Se-52 cathode. Improvement of capacity after 3500 cycles under the influence of high rate cycling, viz., at 2C rate, may be due to the in situ electrochemical activation process of the electroactive material in the NHCS cathode. Thus, the present study validates NHCS as a potential host for Se cathode for longer cycle life applications of Li−Se system with better rate capability along with a recommendation on the amount of Se content in NHCS matrix for specific applications. Impedance Analysis. To obtain the internal resistance and kinetic parameters, electrochemical impedance spectra (EIS) have been recorded for the cell with NHCS/Se-52 and NHCS/ Se-61 cathodes individually (simply cell-52 and cell-61, respectively). EIS measurement has been carried out as a function of DOD and DOC at various voltage levels and at different rates. To start with, the cell was subjected initially to discharge and charge at a C/5 rate and subsequently to rates such as C/5, 1C, 2C, 5C, and 7C. Figure 7a depicts the EIS measurement; Figures S15 and S16 display the Nyquist plots, which consist of depressed semicircle followed by an inclined line. Diameter of the depressed semicircle decreases as a function of depth of discharge, further decreases on subsequent charge, and finally attains minimum value at the end of DOC as evident from Figures S15a and S16a, corresponding to cell-52 and -61 during the first cycle at a C/5 rate. Afterward, the

unfilled pores play a major role in mitigating the volume expansion issues of the cathode during high rate cycling conditions that include 5C, 7C, and 10C rates. Accordingly, NHCS/Se-61 cathode exhibits higher capacity up to 2C rate due to the active participation of a larger concentration of Se and the NHCS/Se-52 cathode, bestowed with more unfilled pores, comparatively admits the volume changes effectively upon high rate conditions beyond the 2C rate. In other words, capacity, which is a direct measure of active concentration of electrode mass stands valid up to the 2C rate only; the available free space or pore volume that sequesters volume expansion decides the behavior above the 2C rate because volume expansion is the major and critical factor at high rates. In contrast, NHCS/Se-72 cathode shows moderate rate capability behavior as given in Figure S13b. Notably, the rate capability study recommends that NHCS/Se-52 and NHCS/ Se-61 are promising cathodes for high and nominal rate applications, respectively, in Li−Se batteries, as mentioned already. With a view to demonstrate the superior capacity of NHCS/Se-52 cathode and its suitability upon extended cycles with high rate, viz., 2C, the cathode has been subjected to a continuous cycling study up to 10 000 cycles. Figure 6e represents the extended cycle life of NHCS/Se-52 cathode and demonstrates the effective restraining of Se and the formed Li2Se by NHCS matrix. The capacity observed at various intervals of cycle number is tabulated (Tables 3). Of the initial discharge capacity, 75% is found to get maintained up to 10 000 26765


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Figure 8. SEM images of NHCS/Se-52 cathode recorded (a−d) before cycling and (e−h) after completing 500 cycles post evidencing the structural integrity upon extended cycles.

after the first cycle. These observations infer that the chainlike Sen molecule (that is formed after the first cycle and found to get maintained throughout cycling) in NHCS exhibits lesser resistance than the ringlike Se8 molecule (initial active material) and Li2Se (discharge product) in NHCS. Comparative charge transfer resistance as a function of DOD and DOC under various C-rates of cell-52 and -61 are shown in Figure 7b. Here again, it proves that RCT exhibits similar behavior at high rates (1C, 2C, 5C, and 10C) compared with that of C/5 rate (after the first cycle). Moreover, similarity in RCT value of the cell-52 for DOD and DOC at high rates indicates the complete electrochemical reversibility of Se in NHCS even at high rates and substantiates the excellent performance observed with respect to rate capability test. However, cell-61 exhibits an increase in RCT with DOD and DOC, when cycled under the influence of high rate, i.e., at 2C, 5C, and 7C rates. Specifically, RCT at the end of discharge and charge is about ∼280 and 140 Ω for all the high rates, viz., at 2C, 5C, and 7C rates, and is ∼l60 and 80 Ω for both C/5 and 1C rates, respectively. The inability to mitigate the volume expansion issues due to lesser unfilled pores of NHCS/Se-61 cathode is responsible for the increase in RCT at high rates; hence, inferior performance is observed with NHCS/Se-61 cathode compared with that of NHCS/Se-52 cathode especially at high rate conditions such as 2C and above. The apparent Li-ion diffusion coefficient (DLi+) of NHCS/ Se-52 and NHCS/Se-61 cathodes has been calculated using eqs 1 and 2:37,38

diameter increases with the depth of discharge and attains a maximum value at the end of discharge, i.e., at 1.0 V. Furthermore, it decreases with the depth of charge and subsequently attains the original value at the end of the charge (Figures S15b−f and S16b−f), irrespective of the current rate. Moreover, the lower frequency region also varies with the depth of charge and discharge, which infers that DOD and DOC have significant dependence on Li-ion transport kinetics in cell-52 and -61.37 Equivalent circuit used to fit the spectrum as shown in Figure S17a, which suggests that depressed semicircle could be correlated to RSEI//CPE1 and RCT//CPE2.17 Warburg impedance (ZW) accounts for linear region in the lower frequency of spectrum, and Rs corresponds to the resistance of the solution or electrolyte. Figure S17b documents that RSEI increases during initial DODs and declines for the rest of select potential points at the first cycle behavior of cell-52 and -61. More specifically, before discharge, RSEI is 227 Ω that decreases to 101 Ω at the end of discharge (1.0 V) and further reduces to 42 Ω at the end of subsequent charge process for cell-52 during the first cycle. Similarly, RSEI is 465, 175, and 27 Ω, respectively, before discharge, fully discharged, and charged state for cell-61 during the first cycle. Interestingly, RSEI exhibits lower and almost constant value (of 30−40 Ω) for cell-52 and -61 after the first cycle. On the other hand, electrolyte resistance (Rs) is small and almost constant (∼2−5 Ω) for cell-52 and -61 during and after the first cycle (Figure S17b). Besides, Figure S17d compares the electrolyte resistance and SEI resistance during extended cycling at 1C, 2C, 5C, and 7C along with C/5 rate as a function of DOD and DOC, which infers that the electrolyte resistance and SEI resistance are almost constant for the select rates (especially after the first cycle) as far as for cell-52 and -61 are concerned. The charge transfer resistance (Rct) declines during the initial cycle (from 170 to 40 Ω and from 300 to 70 Ω for cell-52 and -61 respectively) as displayed in Figure S17c, but RCT increases during the discharge, especially after the first cycle and reaches a maximum value of 102 Ω at the end of DOD. Furthermore, RCT decreases as DOC increases and attains the initial value, i.e., before cycling value of ∼40 Ω, for cell-52 and -61. Notably, the same trend is observed for all the select rates, especially

Zre = K + σwω−1/2

(1)

and D Li+ = R2T 2/2A2 n 4F 4C 2σw 2

(2)

where σw is the Warburg factor (slope of the Zre−ω−1/2 plot) (Ω/s1/2), K is a constant (Ω), ω is the angular frequency (s−1), DLi+ is the Li-ion diffusion coefficient (cm2/s), R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature (K), A is the surface area of electrode (cm2), n is the number of electrons transferred, F is the Faraday constant (96 486 C/ mol), and C is the Li-ion concentration (mol/cm3). 26766


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Figure 9. Electrochemical behavior of NHCS/Se-52 cathode in the potential window of 1−3 V vs Na+/Na. (a) Cyclic voltammogram recorded at a scan rate 0.1 mV/s, (b) galvanostatic charge−discharge cyclability (inset: selective cycles voltage profile), (c) voltage profile, and (d) rate capability of NHCS/Se-52 cathode at various C-rates.

Figures S18 and S19 depict the plots of Zre−ω−1/2, which is used to deduce the Warburg factor (σw). Figures S20 and 7c depict the variation of cell-52 and -61, wherein DLi+ as a function of DOD and DOC measured at regular intervals of voltage and performed at various current rate conditions, viz., C/5 during first cycle, and the subsequent (Figure S20) cycling behavior at C/5 and progressive 1C, 2C, 5C, and 7C (Figure 7c) rates are appended. Diffusion coefficient of 5 × 10−14, 15 × 10−14, and 6 × 10−14 cm2/S and 5 × 10−16, 150 × 10−16, and 8 × 10−16 cm2/s have been calculated for NHCS/Se-52 and NHCS/Se-61 cathodes corresponding to conditions of before discharge, fully discharged, and fully charged state. Nearly one order of variation in DLi+ is observed at the end of charge and discharge for cell-61 especially at higher rates, i.e., 1C, 2C, 5C, and 7C, and the variation is lower for cell-52 (by about 3 times). These variations in DLi+ of cell-61 infers that with the increasing concentration of Se the population of available pores in NHCS decreases and hence hampers the faster diffusion of Li-ions. Detailed impedance studies suggest the following points: (i) Conductivity property of the electrolyte (as inferred by Rs behavior) is not altered during cycling at different rates. (ii) A stable SEI layer is formed during the initial cycle and is maintained throughout extended cycles even at high rates. (iii) The as-formed discharge product, i.e., Li2Se in NHCS, has lower conductivity than Sen molecules in NHCS. (iv) The variation of DLi+ in Li2Se compared with that of Se is 3 and 10 times higher for cell-52 and -61, respectively. Interestingly, the favorable electrode kinetics is believed to be responsible for the extremely high reversibility of NHCS/Se-52 cathode up to 10 000 cycles at 2C rate. Figure S21 compares the Nyquist plot of cycles 1 and 500 of cell-52 and -61 at a C/2 rate, wherein no significant difference is observed. Cell-52 exhibits a constant Li-ion diffusion

coefficient of 5.5 × 10−14 cm2/s in the 500th cycle, thus leading to a negligible difference with respect to the value after first cycle (4.6 × 10−14 cm2/s). In contrast, cell-61 shows almost 50 times higher DLi+ (2.3 × 10−14 cm2/s) after 500 cycles. Thus, EIS measurements suggest that NHCS provides favorable transport kinetics to the impregnated Se with a loading concentration of 52 and 61 wt % even at extended cycles. Post-Cycling Analysis. Figure 8a−d displays the SEM images of NHCS/Se-52 cathode before cycling and Figure 8e−h shows the postcycling SEM images of NHCS/Se-52 cathode, cycled at a 2C rate for 500 cycles. These images confirm that there is no obvious change in the morphology (Figure S22) of the electrode material. Figure S23a−b illustrates the postcycling surface image and EDX spectrum of Li surface. Absence of Se on the Li surface ensures that Li−Se system is free from unwanted redox shuttle reaction. The same could be considered as an authentication to arrive at the fact that 52 wt % Se loading is more desirable than 61 wt % for reasons of enhanced electrode kinetics, effective utilization of Se, and the desired level of mitigation of volume changes upon cycling. The currently observed appreciable electrochemical behavior at 2C rate up to 10 000 cycles is thus understood as a factor of maintenance of structural and morphological features of the NHCS/Se-52 cathode under investigation. Suitability for Na−Se System. As an extended application, the sodium storage property of NHCS/Se-52 cathode has been evaluated in the potential window of 1.0−3.0 V vs Na+/Na in sodium batteries and the performance is shown in Figure 9. Figure 9a displays the CV response recorded at 0.1 mV sweep rate. During the first scan, two reduction peaks appear at 1.76 and 1.24 V (shifted to 1.84 and 1.26 V during further cycling), depicting the formation of Na2Se via an intermediate phase, and 26767


Research Article


loaded NHCS cathode exhibits appreciable capacity, especially beyond 2C rate (ranging from 5C to 10C), and its extended life at 2C rate up to 10 000 cycles has been demonstrated. Furthermore, the study provides a clear understanding of two factors: (a) The capacity of Li−Se system is decided by the extent of utilization of Se confined in the host matrix. (b) Cycle life is governed by the extent of mitigation of volume expansion, which in turn has a major dependence on the availability of unfilled pores to cushion the volume changes. In short, better confinement of optimum concentration of Se and the synergistically adjusted trade-off between filled and unfilled pores are essential to realize the combination of high capacity and longer cycle life. When exploited in Na−Se system, the select NHCS/Se-52 cathode demonstrates its suitability to exhibit high capacity (339 mAh/g) and better tolerance to high rate conditions, viz., 2C, thus providing ample scope for further development. Hence, the study provides newer insight and deeper understanding on the multifunctional capability of NHCS/Se cathode with the optimum concentration of Se.

the subsequent anodic scan contains two peaks located at 1.87 (shifted to 1.99 during further cycling) and 2.15 V due to the reversible formation of Se from Na2Se.12 Since the potential separation between oxidation peaks is smaller (∼0.16 V, especially after first cycling) compared with that of reduction peaks (∼0.58 V), the oxidation peaks may be correlated to the formation of Se molecules in two different environment and/or the formation of Se molecules via intermediate phase. On the basis of the literature reports,16,20,39,40 it is evident that hightemperature (600 °C) infusion of Se into the pores of carbon leads to formation of short-range structure of Se, which follows direct conversion of Se into Na2Se during the sodiation/ desodiation process as indicated by the single CV peak behavior. In contrast, Se impregnated at ≤260 °C follows an intermediate step in the reversible conversion of Se into Na2Se, which is applicable to the present study, wherein Na2Se4 may be the intermediate phase.39 As a result, multiple CV peaks are observed in the present study, which is in agreement with the reported results.39,40 Overlapping of further CV curves documents the cycling reversibility of the electrode. The inset of Figure 9b shows the galvanostatic charge−discharge profiles of cathode cycled at the C/5 rate, wherein a sloping first discharge curve is observed in the potential range of 2.0−1.0 V with a discharge capacity of 656 mAh/g and the corresponding charge capacity of 375 mAh/g. The disparity between charge and discharge capacities could be explained based on similar mechanism discussed in Li−Se battery for the first cycle behavior. Overlapping of subsequent charge−discharge profiles up to 50 cycles indicates the stability of the NHCS/Se-52 cathode in reversibly storing Na. To identify the number of plateaus, dQ/dV versus potential plot is drawn for the second cycle (C/5 rate), and the same is shown in Figure S24. The redox peaks in the plot are exactly matching with those of CV response (with the acceptable potential difference). Yet another reduction peak at 1.62 V is visible in dQ/dV, which may be ascribed to the reduction of Se in two different environments in the NHCS. Figure 9b displays the cyclability of the cathode up to 50 cycles at a C/5 rate. It shows excellent cyclability by way of delivering 352, 362, 373, and 339 mAh/g capacity at cycles 2, 10, 25, and 50 with a Coulombic efficiencies of 99.4, 99.1, 100.1 and 99.7%, respectively. Figure 9c shows similar charge− discharge curves exhibited by the cathode at various C rates, viz., C/5, C/2, 1C, and 2C. The cathode regains 274, 358, and 390 mAh/g capacity when the C-rate is switched back reversibly to 1C, C/2, and C/5, respectively (Figure 9d). Excellent rate capability and cyclability of NHCS/Se-52 cathode demonstrate the possible consideration of its suitability for Na storage in Na−Se system.

■

METHODS

To prepare N-containing carbon spheres with a hollow core and mesoporous shell, initially silica spheres with solid core and mesoporous shell were prepared and carbonized thereafter with polypyrrole followed by HF etching. Synthesis of Silica Spheres with Solid Core and Porous Shell. All the chemicals were used as received without any further purification. Firs, 2.82 g of aqueous ammonia solution (32 wt %, Acros Organics) was added to a mixture of 58.5 g of absolute ethanol (Hayman) and 10 g of deionized water (D.I. H2O, with an electrical resistivity of 18.5 MΩ). To this was added 5.6 g of tetraethyl orthosilicate (TEOS, Sigma-Aldrich), and the solution was kept static for 1 h to yield uniform Stöber silica solution. A mixture of 4.67 g of TEOS and 1.77 g of octadecyltrimethoxysilane (C18TMS, TCI Chemicals) was added dropwise to the above colloidal solution with stirring and kept undisturbed for 1 h. The solvent from the product was removed by heating at 60 °C for 48 h and dried at 100 °C overnight. Silica spheres containing a solid core and a porous shell were obtained by drying and heat treating the powder at 550 °C for 6 h with a heating rate of 1 °C/min in air. Synthesis of N-Containing Carbon Spheres with Hollow Core and Mesoporous Shell and the Formation of Composites with Se. In order to obtain infiltrated FeCl3 (Sigma-Aldrich) inside the pores of silica spheres, a solution of FeCl3 in ethanol was mixed with the as-received silica spheres with solid core and porous shell in the weight ratio of 0.459:1.7 (i.e., 0.27 g of FeCl3 per gram of SiO2). The content was exposed to pyrrole vapor (Sigma-Aldrich) in a closed vessel for 22 h at room temperature. The final product was carbonized at 800 °C for 3 h under Ar atmosphere with a ramping rate of 3 °C/ min. The silica containing carbonized product was treated with 5 wt % HF solution (Rankem) for 10 h. After D.I. H2O wash, the product was dried at 120 °C overnight to obtain N-containing carbon spheres with hollow core and mesoporous shell (NHCS). Selenium (Acros Organics) loaded NHCS (denoted as NHCS/Se-X where X= 52, 61, and 72 wt % of selenium) were obtained by melt diffusion strategy. The required amount of selenium was mixed well with NHCS and kept at 260 °C for 12 h in a closed vessel to obtain NHCS/Se-X composition. Material Characterization. NHCS and the range of NHCS/Se-X compositions were subjected to the following investigations. XRD was recorded with a Bruker D8 Advance X-ray diffractometer (XRD) using Ni-filtered Cu Kα radiation. TGA, Raman spectral studies and electrical conductivity of NHCS and NHCS/Se-X were performed using NETZSCH STA 449F3, Renishaw inVia Raman Microscope using He−Ne LASER and four probe setup (SES Instrument Pvt, Ltd.), respectively. N2 adsorption/desorption isotherm was obtained from Quantachrome Instruments version 3.01, and XPS results were from a MULTILAB 2000 Base system. Morphology was studied using

■

CONCLUSION N-doped hollow core mesoporous shell carbon spheres with a diameter of

Designed Formulation of Se-Impregnated N ... - ACS Publications

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