Designed Formulation of Se-Impregnated N-Containing Hollow Core

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Designed Formulation of Se impregnated Nitrogen containing Hollow Core Mesoporous Shell Carbon Spheres: Multifunctional Potential Cathode for Li-Se and Na-Se Batteries Balakumar Kalimuthu, and Kalaiselvi Nallathamby ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05103 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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ACS Applied Materials & Interfaces

Designed Formulation of Se impregnated Nitrogen containing Hollow Core Mesoporous Shell Carbon Spheres: Multifunctional Potential Cathode for LiSe and Na-Se Batteries Balakumar Kalimuthu and Kalaiselvi Nallathamby* Electrochemical Power Systems Division, CSIR- Central Electrochemical Research Institute, Karaikudi- 630 003, India

ABSTRACT: Nitrogen containing carbon spheres with hollow core and mesoporous shell (NHCS), capable of confining Se as high as 72 wt.% has been demonstrated to exhibit appreciable electrochemical behaviour 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 10000 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. Further, 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. Further, NHCS/Se-52 cathode demonstrates suitability for Na-Se batteries by exhibiting 339 and 219 mAh/g of capacity at

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C/5 and 2C rate respectively. NHCS with select Se concentration could thus be exploited for multifunctional cathode behaviour in Li-Se and Na-Se systems.

KEYWORD: N-doped hollow carbon spheres, Mesoporous nano-shell carbon, cycleability, Li-Se, Na-Se batteries. Introduction Limited availability of non-renewable energy resources and the threatening environmental pollution issues urge 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 the 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 are not only having higher theoretical capacity and energy density, but also exhibit vast scope of practical viability and application by proper electrode design.5-7 In particular, Li-S fulfills the demand on 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). Further, Se possesses inherent advantages such as better electronic conductivity in the order of 10-5 S/cm (as a d-electron containing element) than its congener S (~10-30 S/cm), and weaker shuttle effect, which are in favour of the high rate performance and maximum utilization of electrochemically active Se material.12 Based on 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,


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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 in this connection, the current study assumes importance in providing detailed information about the synthesis-structure-property relationship that plays a pivotal role in identifying potential cathode candidate for Li-Se system. Parallely, yet another feasible and futuristic energy storage strategy based on Na-Se batteries has also been explored with a carefully chosen optimised cathode formulation and demonstrated the multifunctional capability of NHCS-Se cathode through the present study. Basically, electrochemical 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 multi-electron 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 like 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 shows less improvement only.13, 14 In order to overcome these hurdles, porous carbon matrices with meso/microporous nature and candidates such as



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graphene and hybrid structure showing excellent electrochemical performance have been investigated.15-23 Basically, 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 few other 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. 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. Further, smaller size of nitrogen atom and the creation of nanopores in carbon lattices faster the lithium ion transfer kinetics of Se cathode.24-26 Moreover, variation in the local charge densities of the carbon network due to N doping creates a polar nature that facilitates the favourable 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, N-doping provides facile electronic conductivity even at high rates and upon 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


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cathode, wherein higher Se confinement in its amorphous form, lesser concentration of crystalline Se found to be intact with the NHCS, individual and perfectly formed nitrogen containing hollow core mesoporous shell carbon without any undesirable agglomeration and the resultant cathode with comparatively lesser polarization behaviour have been demonstrated. Nitrogen containing hollow carbon spheres (NHCS) with mesoporous shell has been synthesized using silica hard template method, wherein pyrrole serves as a heteroatom (N) containing carbon precursor.29-30 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. On the other hand, 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. Based on these structural advantages, NHCS is believed to increase the actual utilization of active material. Interestingly, NHCS of the present study admits Se loading up to 72 wt.% and exhibits superior electrochemical performance with respect to 52 and 61 wt.% Se loading, due to reasons based on the trade-off between filled and unfilled pores in cushioning the volume expansion to the desired level. In particular, the nano regime 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 10000 cycles at 2C rate, which is noteworthy. As an extended application, the optimized composition of 52 wt.% of Se @ NHCS has been explored for its possible cathode behaviour in Na-Se batteries also. 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 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 as stober silica spheres, were synthesised by the hydrolysis and cross linking of TEOS in ammonia containing water-ethanol solvent. Subsequent co-condensation of TEOS and C18-TMS porogen with the as-formed 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. Porogen 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 nitrogen 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 1(a) 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 centred 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 matches with the pattern of hexagonal Se (ICDD: 06-0362). 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 an


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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 atm. 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 1(b) shows the N2 adsorption and desorption isotherm of NHCS and NHCS/Se-52 and 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 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 m2/g and 39 m2/g for NHCS/Se-52 and 61 composites respectively. Pore size distribution curve of NHCS (Figure 1(c)) 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 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



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the active material during cycling, based on the aforesaid trade-off factor related to filled/unfilled pores vs. volume change.

Raman behaviour 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 in different concentration in the NHCS composites shows no significant deviation of ID/IG value from that of pristine NHCS (ID/IG value of 1.03, 0.99 and 1.03 corresponding to NHCS/Se-52, 61 and 72), 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, NHCS/Se52, 61 and 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 composites were analysed using FESEM/HRTEM and the captured images are shown in Figure S3, Figure 2(a-d) and Figure 2(e-k) respectively. Figure S3(a-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


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shell thickness of about 40 ± 5 nm. Amorphous nature of the template is evident from the corresponding SAED pattern (Figure S3(e)). Figure 2(a) shows the FESEM image of NHCS that clearly visualizes the presence of uniformly distributed nano carbon spheres. HRTEM images presented in Figure 2(b-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, containing hollow core structure and porous shell morphology, as revealed by Figure 2(b). Further, Figure 2(d) 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 S4(a) and the corresponding SAED pattern (inset of Figure 2(d)) shows the amorphous nature of NHCS. Abundantly available pores on the shell along with the hollow core structure offers ample scope for the easy percolation of electrolyte and to facilitate facile Li+ transport, which is interesting. HRTEM images of NHCS/Se-52 composite captured under lower magnification (Figure 2(e-g)) evidence the indistinguishable difference found with that of pristine NHCS. Figure 2(h) and (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 2(j)) and Figure 2(k) contain the characteristic pattern of fringes pertinent to carbon in NHCS. HRTEM image captured at the edge of the shell (Figure 2(k)) evidences the presence of pores in NHCS. SAED pattern of NHCS/Se-52 (inset of Figure 2(k)) 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 reside within the mesopores of NHCS. FigureS5 shows the STEM image and elemental mapping of pristine NHCS, which demonstrates the uniform distribution of nitrogen in the carbon shell. Further, STEM mapping of NHCS/Se-52 composite Figure 2(m-p) evidences the homogeneously distributed Se, confined in the porous shell of NHCS. Significant graphitic



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nature of porous NHCS with the uniformly impregnated Se in the pores will be beneficial in terms of effective utilization of Se, high rate tolerance of the composite cathode and 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. XPS survey spectrum of NHCS (Figure S6(a)) contains three peaks at 289.3, 400.7 and 531.6 eV, revealing the presence of carbon, nitrogen and oxygen 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 Figure 3(a-f) and S6(b). 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) nitrogen to the extent of 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. On the other hand, 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, deconvoluted Se 3d spectrum contains 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 to be (Figure 3(e)) 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


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energy of N-IV and N-V 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, linear chain like Sen molecule, formed after the first cycle exhibits higher binding energy in Se 3d core level spectrum, due to the interaction between chain like Sen with 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 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 chain like Sen molecule, which is believed to be formed during the melt diffusion process itself. In addition, the higher binding energy peak of Se peak 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, 61 and 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, 61 & 72 cathodes were recorded in the potential window 1.0 to 3.0 V with a scan rate of 0.1 mV/s (Figure 4 and S8). Figure S7 depicts the CV behaviour of pristine NHCS cathode and signifies the insignificant response after first cycle, which is not unusual. First cathodic scan of NHCS/Se-52 cathode (Figure 4(a)) demonstrates two reduction peaks at 2.35 and 1.72 V, corresponding to the conversion of cyclic Se8 into chain like (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 chain like Sen



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molecule.15, 17 Subsequent cycles show identical behaviour 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 length.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 4(b)) 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 chain like 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 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), 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 72 cathodes. The observed identical behaviour of NHCS/Se-52 and 61 cathodes upon initial oxidation and subsequent cycling indicates the involvement of similar mechanism, governing the reversibility of said cathodes. On the other hand, 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 to 3.0 V. The capacity calculation was made based on the amount of Se in the composite cathode (1C rate =675


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mA/g). Figure S9 corresponds to the charge/discharge behaviour of NHCS, wherein inferior capacity of ~ 40 mAh/g has been observed. Similarly, inferior Coulombic efficiency of < 30% has been observed, which upon subsequent cycles improves gradually to reach > 90% after 40 cycles. However, the data carries no great significance, as the experiment has been done with a scientific curiosity to understand the behaviour of NHCS, as a preliminary study. Figure S10 displays the first discharge-charge curve of NHCS/Se-52 and 61 cathodes under C/5 rate. Initial irreversible discharge plateau that appears in the region of 2.35 to 2.20 V gives rise to a capacity of ~20 mAh/g, which may be due to the conversion of cyclic Se8 into chain like 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 chain like 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 5(a) shows the charge and discharge profile of NHCS/Se-52 cathode, which coincides with the CV behaviour. 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. 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 rate and the corresponding voltage profile is shown in Figure S11(a), (b) and (c). Interestingly, NHCS/Se-52 cathode exhibits perfect overlapping of charge and discharge curves up to 1000 cycles under C/2 rate.



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On the other hand, potential separation between charge and discharge plateaus, 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 5(b). It is quite interesting to note that the initial polarization that starts at 0.32 V in the first cycle is found to get reduced to the extent of 0.28 V (50th cycle) and 0.23 V (500th cycle) at C/5 rate, which is an evidence for the better confinement of Se in NHCS/Se-52 cathode. Similarly, the polarization decreases from 0.34 V (1st cycle) to 0.25 (1000th cycle), when cycled under 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 1C rate. More interestingly, 2C rate experiences much reduced polarization behaviour (0.65 V in the first 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 favoured, as far as NHCS-52 cathode is concerned, which is the highlight of 52 wt.% Se loaded cathode investigated through this study Figure 5(c) depicts the capacity vs. cycle number behaviour of NHCS/Se-52 cathode studied as a function of rates such as C/5, C/2, 1C & 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 547 mAh/g with an excellent Coulombic efficiency of 98.9% has been observed, thus indicating the cycleability, retention capability and stability of NHCS/Se-52 cathode upon extended cycles. Similarly, 90% of the initial capacity is found to get retained up to 1000 cycles, when cycled under C/2 rate. Further, excellent retention of


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capacity to the extent of 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 at extended cycling and rate conditions. Similarly, the electrochemical performance of 61 wt.% of Se loaded NHCS was evaluated under different rated current conditions to understand the feasibility of exploring the electrode for high energy density applications of Li-Se system. Figure S10 shows the first discharge-charge curve of NHCS/Se-61 cathode under C/5 rate. As observed in CV, first discharge curve consists of one additional irreversible plateau at 2.49 V with a capacity of < 10 mAh/g along with two discharge plateaus as in the case of NHCS/Se-52 cathode. Herein, the plateau in the range of 2.35 to 2.20 V contributes to ~60 mAh/g (higher than NHCS/Se-52 cathode) to the total capacity of 1050 mAh/g. First cycle charging shows typical single plateau behaviour with a capacity of 569 mAh/g and Coulombic efficiency of 54.2%. Voltage profile of NHCS/Se-61 cathode under C/5 rate is shown in Figure 5(d). Two discharge plateau and single charge plateau are observed from 2nd cycle onwards, corresponding to NHCS/Se-61 cathode, which is identical to that of NHCS/Se-52 cathode. However NHCS/Se-61 cathode delivers higher discharge and charge capacity than NHCS/Se-52 cathode. Figure S12(a-c) shows the profile of NHCS/Se-61 cathode cycled at C/2, 1C and 2C rates. Appearance of identical plateaus with increasing polarization behaviour has been observed for all the rates after 100 cycles. Figure 5(e) depicts the variation of polarization as a function of cycle number, corresponding to different rates. Herein, NHCS/Se-61cathode cycled at C/5, C/2, 1C and 2C rate shows polarization to the extend of 0.26, 0.26, 0.32 and 0.47 V during 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.



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Interestingly, it is found that rate of cycling affects the polarization to a greater extent up to 100 cycles. i.e. for moderate rates such as C/5 and C/2, decrease in polarization is observed and it is almost constant that begins to increase with higher (1C and 2C) rates. Beyond 100 cycles, it shows higher polarization than 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 on Se against continuous volume changes anticipated during cycling. It is worth to mention here 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 behaviour beyond 100 cycles of NHCS/Se-61 cathode is correlated to the higher concentration of Se (than the preferred optimum amount), which in turn is responsible for the insufficient buffering of volume expansion occurring upon cycling. i.e. high Se loading leaves 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 5(f) summarizes the cycling behaviour 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 excellent reversibility of higher Se loading (61 wt.%) could be understood from its capability to retain 93 and 83% of initial capacity at the 250th and 500th cycle respectively with about 100% approaching Coulombic efficiency. From Table 2, it is found that almost no loss in capacity up to 250 cycles is observed and the cathode retains about 90% of initial capacity at the 500th cycle under C/2 rate, 97 and 82% of the initial capacity under 1C rate and 96 and 47% of the initial capacity under 2C rate at the 100th and 500th cycle respectively. Observation related to >80 and 45% of the capacity retention at 1 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


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of hollow core and mesoporous carbon spheres in ensuring the appreciable performance of high energy density Li-Se system with extended cycle life behaviour. Hence, the observed inferior electrochemical behaviour 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 61 cathodes for select characterizations. Rate capability and Extended cycling Rate capability study of NHCS/Se-X (X=52 and 61) cathodes was performed to further understand the role of NHCS in enabling the electrochemical utilization of Se and the result is summarized in Figure 6(a-d) and Figure S13(b). Figure 6(a) and (b) display the voltage profile of NHCS/Se-52 and 61 cathodes under the influence of C/5, C/2, 1C, 2C, 5C, 7C and 10C rates. Retention of discharge-charge plateau even at 10C rate for NHCS/Se-52 cathode and up to 7C rate for NHCS/Se-61 cathode proves the promising advantages of NHCS that acts as a host matrix for Se cathode. Figure 6(c) compares the polarization behaviour of NHCS/Se-52 and 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 rate have been observed. Polarisation is insignificant up to 1C rate and a gradually increasing polarization behaviour with the increasing C rates, such as 2, 5, 7 and 10C has been noticed for both 52 and 61 cathodes. It is evident from Table 3(a) that the capacity of NHCS/Se-61 cathode is superior with respect to 52 wt.% loaded cathode up to 2C rate, whereas the capacity at 5C, 7C and 10C are found to be superior with respect to 52 wt.% Se loading. This observation leads to an understanding that Se present in pores plays a vital role in deciding the rate capability behaviour up to 2C rate and the unfilled pores that play a major role in mitigating the volume expansion issues of the cathode dominate during high rate cycling conditions that include 5, 7 and 10C rates. Accordingly, NHCS/Se-61



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cathode exhibits higher capacity up to 2C rate due to the active participation of larger concentration of Se and the NHCS/Se-52 cathode, bestowed with more population of unfilled pores comparatively admits the volume changes effectively upon high rate conditions beyond 2C rate. In other words, capacity, which is a direct measure of active concentration of electrode mass stands valid up to 2C rate only; the available free space or pore volume that sequesters volume expansion decides the behaviour above 2C rate, because volume expansion is the major and critical factor at high rates. On the otherhand, NHCS/Se-72 cathode shows moderate rate capability behaviour as given in Figure S13. Notably, the rate capability study recommends that NHCS/Se-52 and 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 10000 cycles. Figure 6(e) 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(b)). 75% of the initial discharge capacity is found to get maintained up to 10000 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.


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Impedance analysis To obtain the internal resistance and kinetic parameters, electrochemical impedance spectrum (EIS) has been recorded for the cell with NHCS/Se-52 and 61 cathodes individually (hereafter will be denoted as 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 C/5 rate and subsequently to rates such as C/5, 1C, 2C, 5C and 7C.

Figure 7(a) depicts the EIS

measurement upon Figure S15 and S16 display the Nyquist plots, which consists of depressed semicircle followed by an inclined line. Diameter of the depressed semicircle decreases as a function of depth of discharge and further decreases on subsequent charge and finally attains minimum value at the end of DOC as evident from Figure S15(a) and S16(a) corresponding to cells-52 and 61 during the first cycle at C/5 rate. Afterwards, the diameter increases with the depth of discharge and attains a maximum value at the end of discharge (i.e. at 1.0 V). Further it decreases with the depth of charge and subsequently attains the original value at the end of the charge (Figure S15(b-f) and S16(b-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 cells-52 and 61.37 Equivalent circuit used to fit the spectrum is shown in Figure S17(a), 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 S17(b) documents that RSEI increases during initial DODs and declines for the rest of select potential points at the first cycle behaviour of cells-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



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subsequent charge process for cell-52 during first cycle. Similarly, RSEI is 465, 175 and 27 Ω respectively before discharge, fully discharged and charged state for cell-61 during first cycle. Interestingly, RSEI exhibits lower and almost constant value (of 35 ±5 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 cells-52 and 61 during and after the 1st cycle (Figure S17(b)). Besides, Figure S17(d) 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 cells-52 and 61 are concerned. The charge transfer resistance (Rct) declines during the initial cycle (from 170 to 40 Ω and 300 to 70 Ω for cell-52 and 61 respectively) as displayed in Figure S17(c). But, RCT increases during the discharge, especially after the first cycle and reaches a maximum value of 102 Ω at the end of DOD. Further, 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 after the first cycle. These observations infer that chain like Sen molecule (that is formed after the first cycle and found to get maintained throughout cycling) in NHCS exhibits lesser resistance than ring like 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 cells-52 and 61 are shown in Figure 7(b). Here again, it proves that RCT exhibits similar behaviour at high rates (1, 2, 5 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. On the other hand, 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,


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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 rate and is ~l60 and 80 Ω for both C/5 and 1C rate respectively. The inability in mitigating the volume expansion issues due to lesser unfilled pores of NHCS/Se-61 cathode is responsible for the increase in RCT at high rates and 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 co-efficient ( ) of NHCS/Se-52 and 61 cathodes has been calculated using Equation 1 and 2:37, 38

= + ---(1) and

= /2 --- (2)

Where, - Warburg factor (slope of the Zre - ω-1/2 plot) (Ω s1/2 ), K- constant (Ω), ω angular frequency (s-1), – Li-ion diffusion co-efficient, R - gas constant (8.314 J mol-1 T1

), T - absolute temperature (K-1), A – surface area of electrode (cm2), n - number of electron

transferred, F - Faraday constant (96486 C/mol) and C- Li-ion concentration (mol/cm3).

Figure S18 and S19 depict the plots of Zre - ω-1/2, which is used to deduce the Warburg factor ( ). Figure S20 and 7(c) depict the variation of cells-52 and 61, wherein 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 behaviour at 1C, 2C, progressive 5C and 7C (Figure 7(c)) rates are appended. Diffusion coefficient of 5 x10-14, 15 x10-14 and 6 x10-14 cm2/S and 5 x10-16, 150 x10-16 and 8 x10-16 cm2/s



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have been calculated for NHCS/Se-52 and 61 cathodes corresponding to conditions such as before discharge, fully discharged and fully charged state. Nearly one order variation in 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 (about 3 times). These variations in 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 behaviour) 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 in Li2Se compared with that of Se is 3 and 10 times higher for cells-52 and 61 respectively. Interestingly, this favourable electrode kinetics is believed to be responsible for the extremely high reversibility of NHCS/Se-52 cathode up to 10000 cycles at 2C rate.

Figure S21 compares the Nyquist plot of 1st and 500th cycle of cell-52 and 61 at C/2 rate, wherein no significant difference is observed. Cell-52 exhibits a constant Li-ion diffusion coefficient of 5.5 x 10-14 cm2/s in the 500th cycle, thus leading to a negligible difference with respect to the value after 1st cycle (4.6 x 10-14 cm2/s). On the other hand, cell-61 shows almost fifty times higher (2.3 x 10-14 cm2/s) after 500 cycles. Thus, EIS measurements suggest that NHCS provides favourable transport kinetics to the impregnated Se with a loading concentration of 52 and 61 wt.% even at extended cycles.

Post cycling analysis


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Figure 8(a-d) displays the SEM images of NHCS/Se-52 cathode before cycling and Figure 8(e-h) shows the post cycling SEM images of NHCS/Se-52 cathode, cycled at 2C rate for 500 cycles. These images confirm that there is no obvious change in the morphology (Figure S22) of the electrode material. Figure S23(a-b) illustrates the post cycling 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 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 behaviour at 2C rate up to 10000 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, sodium storage property of NHCS/Se-52 cathode has been evaluated in the potential window 1.0 to 3.0 V vs. Na+/Na in sodium batteries and the performance is shown in Figure 9. Figure 9(a) displays the CV response recorded at 0.1 mV sweep rate. During 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 phaseand 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. Based on the literature reports 16, 20, 39, 40, it is evident that high temperature (600 °C) infusion of Se into the pores of carbon leads to formation of



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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 behaviour. On the other hand, 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. Inset of Figure 9(b) shows the galvanostatic chargedischarge profiles of cathode cycled at C/5 rate, wherein a sloping first discharge curve is observed in the potential range of 2.0 to 1.0 V with a discharge capacity of 656 mAh/g, and the corresponding charge capacity is 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 behaviour. 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 vs. potential plot is drawn for the 2nd 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 environment in the NHCS. Figure 9(b) displays the cycleability of the cathode up to 50 cycles at C/5 rate. It shows excellent cycleability by way of delivering 352, 362, 373 and 339 mAh/g capacity at the 2nd, 10th, 25th and 50th cycle with a Coulombic efficiency of 99.4, 99.1, 100.1 and 99.7% respectively. Figure 9(c) 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 9(d)). Excellent rate capability and cycleability of NHCS/Se-52 cathode demonstrates the possible consideration of its suitability for Na storage in Na-Se system.


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CONCLUSION

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

Designed Formulation of Se-Impregnated N-Containing Hollow Core

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