Newer polyanionic bio-composite anode for sodium ion batteries

Journal of Power Sources 340 (2017) 401e410

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Newer polyanionic bio-composite anode for sodium ion batteries Saravanan Karuppiah a, b, Suganya Vellingiri a, Kalaiselvi Nallathamby a, b, * a b

CSIR-Central Electrochemical Research Institute, Karaikudi 630 006, Tamilnadu, India AcSIR e Academy of Scientific & Innovative Research, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A simple, facile, effective and scalable green composite anode is proposed.
Na3V2(PO4)3/HHC is a newer biocomposite anode formulation.
Appreciable reversible capacity of 125 mAh g1 at 0.05 A g1 up to 100 cycles.
Excellent rate capability at 2 A g1 for 500 cycles has been demonstrated.
HHC ensures better electronic conductivity and cushioning effect.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2016 Received in revised form 9 November 2016 Accepted 25 November 2016

NASICON frame work Na3V2(PO4)3 (NVP), wrapped by nitrogen and sulfur doped bio-carbon matrix derived from human hair (HHC) has been investigated for its anode behavior in SIBs. Basically, NVP is bestowed with a crystal structure of 3D open framework and a moderate theoretical capacity of 118 mAh g1, which are the twin advantages and motivation behind the selection of this material. Prepared through a simple, scalable and facile method, the key problems associated with pristine NVP electrode material, such as inferior conductivity and severe volume change have been mitigated to a great extent through the formation of a composite containing HHC. Herein, HHC is a cheap and eco-friendly composite additive, obtained from a universal bio-waste, viz., human hair and hence NVP/HHC qualifies itself as a green composite. Interestingly, NVP/HHC-10 (in-situ) and NVP/HHC-20 (ex-situ) anodes show excellent electrochemical performance in terms of cycling stability up to 500 cycles and rate capability @ 2 A g1, which are superior than similar category NVP anodes reported in the literature. Further, post cycling structure and morphology of NVP/HHC composite anodes evidence the appreciable stability bestowed with the select composition, which is found to get maintained upon extended cycles and even after rate capability test. © 2016 Elsevier B.V. All rights reserved.

Keywords: Na3V2(PO4)3 Human hair derived carbon Anode Sodium-ion batteries

1. Introduction Rechargeable lithium ion batteries (LIBs) remain as fascinating and dominating electrochemical energy storage systems (EES) in

* Corresponding author. CSIR-Central Electrochemical Research Institute, Karaikudi 630 006, Tamilnadu, India. E-mail address: [email protected] (K. Nallathamby). http://dx.doi.org/10.1016/j.jpowsour.2016.11.095 0378-7753/© 2016 Elsevier B.V. All rights reserved.

the global market share over the past two decades, due to their superior properties like high open circuit voltage, high energy and power density and long cycle life [1e3]. However, intricacies related to the unevenly distributed and/or limited availability and rising price of lithium (Li) resources project potential problems against the commercial exploitation and global acceptance of LIBs into high energy fields, such as electric vehicles and grid storage [4]. Thus, development of alternative EES is not only desirable but also

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necessary. Sodium-ion batteries (SIBs) are inviting considerable attention as a promising alternate to LIBs due to the abundant availability of resources and low cost nature of sodium (Na). Furthermore, Na and Li share close similarity in many chemical properties. Especially, intercalation chemistry of SIBs is similar to those of LIBs, which in turn has advanced rapid developments in the field of SIB technologies [5e8]. Despite the rapid development of various high performance cathode materials for SIBs (metal oxides, phosphates and ferrocyanides) [9e17], the development of anode materials for the same remains in the nascent stage. It is in this regard, various carbonaceous materials have been investigated for their electrochemical performance towards SIBs as anode [18e21]. However, their performance in terms of cycleability and storage capability is relegated than that of the corresponding lithium counterparts. Because, the radius (0.98 Å) of Na is much larger than that of Li (0.69 Å), which leads to the lower diffusion of Naþ during the intercalation of the same into the electrode. In particular, the aforementioned limitation of larger ionic radius leads to huge volume changes, especially when Naþ is inserted into/extracted from the host materials, thus causing severe structural degradation as well as the consequence of exhibiting poor cycleability and rate capability [4]. Hence, major and ubiquitous attention is warranted towards finding suitable anode materials for SIBs with higher storage performance, long cycle life and outstanding rate capability. With this background, materials possessing unique structures, such as open and stable three-dimensional (3D) framework containing open diffusion pathways with large tunnels and larger interstitial spaces are expected to overcome the said issues of SIBs by offering perturbation free intercalation/de-intercalation process [4,22]. Polyanion based electrode materials exhibit 3D open frameworks due to strong covalent bonding between oxygen in polyanion polyhedral, which results in high thermal stability. Within the family of polyanionic compounds, an open Na NASICON superionic conductor framework holding Na3V2(PO4)3 (NVP) has attracted tremendous attention, owing to its high sodium ion conductivity, high operating voltage (3.4 V) and high theoretical energy density (400 Wh kg1) [14,23]. It is important to mention here that the material with NASICON structure is bestowed with higher conductivity and chemical diffusion co-efficient of Naþ compared to Liþ [23,24]. However, the practical use of NVP is still hindered due to its low electronic conductivity, and hence the realization of 100% theoretical capacity (117.6 mAh g1) in practical applications or device assembly is arduous with respect to the NASICON type material [25e28]. In other words, the theoretical capacity of NVP has not been realized so far even at comparatively lower rates, thus presenting a major drawback to consider it further for practical applications requiring high rate capability. Strategies to increase sodium storage performance by way of increasing the charge transfer kinetics in NVP involve coating of NVP particles with highly conductive and interconnected carbon frameworks and reducing the size of NVP particles to shorten the Naþ ion diffusion length [14,26,29e31]. Hence, construction of efficient and electronically conducting network is highly essential to form an electron wiring pathway among the NVP particles to increase their capacity through effective active mass utilization and to ensure rate capability behavior with faster transport of host sodium ions within the structure. In addition, advantages due to the interconnected carbon framework that acts as a buffering matrix to accommodate volume changes (strain) with the repeated Naþ insertion and extraction during charge and discharge process could also be reaped out. In other words, the structural integrity of the material is required to be maintained throughout the process, prior to its recommendation for practical applications and the same is believed to be

achieved through the provision of conducting carbon network. Towards this direction, many reports have demonstrated the superior electrochemical performance of porous carbon wrapped/ coated NVP nanoparticles (as cathode) in SIBs [4,22,26,30,32e56]. In particular, carbon coated NVP nanoparticles in a porous graphene network is reported to show excellent electrochemical performance in terms of capacity as well as rate capability [40,42,49,55]. Inspired by these approaches, we have revisited our previous work to provide a suitable link to the currently discussed NVP related materials chemistry for its translation in to device application. In one of our earlier attempts, we have synthesized dual hetero atom containing randomly oriented graphene sheet like carbon (HHC) from human hair waste and exploited the same as an eco-benign and cheap additive to obtain metal oxide nanocomposite with a view to replace the costly carbon derivatives such as graphene and rGO, deployed popularly as composite additives. It is very important to mention here that the combination of HHC with metal oxide shows the highest ever reported reversible lithium storage capacity and rate capability [57]. Further, it is believed that the obtained superior electrochemical performance is associated with the unique properties of HHC including its improved electronic conductivity, faster lithium-ion transport along with the action of buffering matrix to reduce the severe volume expansion of metal oxide anode during cycling process. Albeit this success, HHC alone has also been reported by our group as an anode material for LIBs and SIBs [58,59]. Thanks to the success/promising results of our previous work in LIBs, which really inspires us this time to explore HHC as a composite additive for NVP anode and its application in SIBs. Apart from its unique and randomly oriented dual hetero atom doped porous graphene sheet like structure of HHC, the following appealing features motivated us to consider its incorporation to advance the performance of NVP by a making composite, viz., NVP/ HHC anode. (i) Mesoporous network and graphene sheet like structure of HHC may enable the rapid ion diffusion and help to overcome the strain induced by volume changes anticipated during sodiation and desodiation process. (ii) Complete wrapping of NVP nanoparticles by HHC will prevent the particle agglomeration and will improve the electronic conductivity of the electrode. (iii) Highly conducting nature of HHC will facilitate the fast electron transfer within the electrode and to the current collector. (iv) Presence of dual heteroatom will boost the electrochemical performance by offering more number of active sites (defects) and modulate the electronic conductivity to the required level. In this approach, NVP nanoparticles are wrapped in the graphene sheet like carbon (denoted as NVP/HHC) via. simple sonication followed by calcination at low temperature. Firstly, NVP has been synthesized via. combustion technique. NVP/HHC composite has been prepared by adding the required amount of HHC with NVP nanoparticles either internally or externally as a compositing additive. The as-prepared NVP/HHC composites have been examined for their anode behavior in SIBs along with pristine NVP anode. The custom designed NVP/HHC composite anode of the current study exhibits excellent rate capability (30 mAh g1 at 2000 mA g1 for 500 cycles) and outstanding cycling stability (500 cycles) compared with that of the pristine one, indicating the superiority and suitability of composite anode for practical applications. Hence, it is understood that the dual hetero atom doped randomly oriented graphene sheet like carbon coating offers variety of advantages to

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2. Experimental

BrunauereEmmetteTeller (BET) method (Autosorb IQ2 Quantachrome India) at liquid nitrogen temperature. Electrochemical studies were carried out using Biologic VMP3 multichannel potentiostat and ARBIN chargeedischarge cycle life tester. Electrochemical impedance (EIS) measurements were carried out on a Biologic VMP3 multichannel potentiostat. The Nyquist plots were recorded potentiostatically by applying an AC voltage of 5 mV amplitude in the 100 kHz to 10 mHz frequency range.

2.1. Synthesis of Na3V2(PO4)3 (NVP)

4. Electrode fabrication

All chemical reagents were of analytical grade and used as received without any further purification. Solution assisted combustion (SAC) method was adopted to prepare NVP using oxalic dihydrazide (98%, Alfa Aesar) as fuel along with the combination of precursors viz., NaNO3 (98%, Alfa Aesar), V2O5 (99.6%, Alfa Aesar) and NH4H2PO4 (99%, Merck). Primarily, the reactants were added to distilled water with magnetic stirring and heated to 100  C to get a homogeneous solution. The process of stirring and heating was continued until it forms a foamy mass. The obtained foamy mass was dried in hot air oven at 120  C for overnight and heated to 300  C for 4 h in a furnace at a heating rate of 5  C per minute with Ar/H2 (90:10) gas mixture. Further, the powder was heated to 850  C for 8 h with an intermittent grinding to obtain the final product NVP.

The electrode was fabricated from a mixture consisting of NVP, super P carbon black (additive) and polyvinylidene fluoride (binder) in the weight ratio 70:20:10. The mixture was ground and mixed with N-Methyl pyrrolidin-2-one to form a slurry. The resultant slurry was coated on a thin copper foil and dried at 80  C, followed by 120  C for 2 h. Glass fiber was used as the separator and the electrolyte was 1 M NaClO4 dissolved in 1:1 v/v EC:PC. 2032 coin cells were assembled in an Argon-filled glove box and crimp sealed.

sodium chemistry also. As expected, HHC provides continuous carbon network, acts as a cushion to buffer volume changes of NVP and facilitates easy percolation of electrolyte and faster transport of electron through its mesoporous nature, thus qualifies itself as a suitable additive to NVP in recommending its suitability as a potential anode for high capacity and high rate SIB applications.

2.2. Synthesis of human hair derived carbon (HHC) N and S doped human hair derived carbon (HHC) was synthesized using a simple method. To obtain HHC, human hair was collected from a healthy volunteer of our research group. To ensure quality and consistency, hair was collected from the same person at every time. Synthesis of HHC involves a simple approach as reported in our previous study [58]. 2.3. Preparation of NVP/HHC composite To prepare the ex-situ composite, the as-prepared NVP and HHC were transferred individually to ethanol solution and sonicated for about 60 min separately. The dispersed NVP in ethanol was added drop wise into HHC solution and sonicated further for 30 min to ensure the complete wrapping of NVP particles by HHC sheets. The solution was dried in hot air oven at 80  C for 8 h. Subsequently, the mixture was heated in a furnace at 450  C for 2 h in argon atmosphere with a heating rate of 2  C per minute to ensure better adherence between NVP nanoparticles and with the graphene sheet like carbon. For in-situ carbon coating, HHC was added along with the precursors during the course of NVP synthesis itself. For clarity, the composite materials will be hereafter denoted as NVP/ HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/HHC-20 (ex-situ). 3. Characterization Phase characterization was done by powder X-ray diffraction technique on a PANalytical X'pert PRO X-ray diffractometer using Ni-filtered Cu Ka radiation (l ¼ 1.5406 Å). Particle size, surface morphology and the presence of carbon coating were characterized by Scanning Electron microscopy (SEM, JEOL JSM 6480 LV System) and High Resolution Transmission Electron microscopy (HRTEM). Raman spectrum was recorded using a BRUKER RFS 27 stand alone FT-Raman spectrometer in the 300e2500 cm1 spectral range and the laser source is Nd:YAG 1064 nm. Further, thermogravimetric analysis (TG/DTA) of the samples was carried out on a diamond TG Thermo e analyzer to identify the amount of carbon present in the sample. The specific surface area was measured using the

5. Results and discussion 5.1. Physical characterization Fig. 1 shows the powder XRD pattern of as-prepared pristine NVP, NVP/HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/HHC-20 (ex-situ), individually. All the diffraction peaks can be exactly indexed to a rhombohedral unit cell with the R3c space group (JCPDS card No. 00-053-0018), sans impurities and agreeing well with the previous reports [4,22,40e43]. The calculated lattice parameter values [a ¼ 8.72 (Å) and c ¼ 21.82 (Å)] are in good agreement with the reported values in the literature [4,30,35]. There is no apparent evidence for the presence of carbon related peaks in the NVP/HHC pattern, mainly due to the amorphous nature of HHC. More evidently, presence of HHC was confirmed by the recorded Raman spectrum (Fig. S1) and quantified from TGA studies (Fig. S2). Raman spectrum was recorded to investigate the surface features of pristine NVP and NVP/HHC composites (Fig. S1). Pristine NVP exhibits several bands below 1100 cm1, corresponding to the Raman fingerprint characteristics of NVP. Bands located at 1000 and 459 cm1 are attributed to the stretching vibrations of PO4 [22,41,42]. For NVP/HHC composites, two characteristic bands around 1345 (D-band, disorder-induced phonon mode) and 1605 cm1 (G-band, graphite band) are obviously seen, which can be assigned to the typical Raman features of carbon materials [22,41,42,45]. Fig. S2 shows the TGA results, from which the carbon content in the pristine NVP, NVP/HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/HHC-20 (ex-situ) composites has been calculated as 5, 10, 11 and 14% respectively. Noticeably, the TGA curves of NVP and NVP/HHC composites exhibit weight gain signature after 500  C, which is due to the oxidation of V3þ in the NVP to V4þ and V5þ. This observation is in good agreement with the reported results [43,45]. In other words, carbon (residual or intentionally added) present in NVP gets oxidized initially and when the temperature exceeds 500  C, vanadium tends to undergo oxidation irrespective of the presence and amount of coexisting carbon and thereby leading to a resultant weight gain behavior above 500  C in TGA studies, which is not unusual. The nitrogen isotherm adsorption/desorption curves and the pore size distributions of pristine NVP and NVP/HHC composites are shown in Fig. S3 and S4. All the four samples exhibit type IV isotherm with a type-H3 hysteresis loop, demonstrating the presence of micro/mesoporous structure within a wide range of 2e200 nm. Peak at 2e3 nm that has been

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Fig. 1. XRD pattern of pristine NVP and NVP/HHC composites.

observed in the pore-size distribution curve of the NVP/HHC composite samples could be associated with the presence of HHC, incorporated as a composite additive. The porous structure in general is beneficial for the penetration of the electrolyte and it expands the contact area between the electrode and electrolyte, both of which can facilitate faster diffusion of Naþ during the sodiation/desodiation process related to the present study. This in turn is expected to contribute favorably to improve the electrochemical performance and hence the BET derived results assumes importance. Accordingly, the specific surface area values of NVP/ HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/HHC-20 (ex-situ) were calculated to be, 47.93, 57.07 and 120. 27 m2 g1 respectively, which are much higher than that calculated for pristine NVP without HHC (4.16 m2 g1). Fig. S5 reveals the scanning electron microscopy images of pristine NVP, NVP/HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/ HHC-20 (ex-situ) powders. It is evident from the figure that pristine NVP shows a network of aggregated particles with an irregular morphology and shape (Fig. S5a). On the other hand, such irregularly shaped NVP particles are distributed in the porous and graphene sheet like structure of HHC in the composites, which is obviously seen from Fig. S5 (b-d). It is well known that such a porous morphology is beneficial to promote easy penetration of electrolyte and a possible increase in contact area between the electrode and electrolyte, which might be helpful in improving the electrochemical performance of the electrode material, especially at high rates [46]. HRTEM analysis was used to investigate the morphology and internal architecture of pristine NVP and NVP/HHC composites, and the captured images are furnished in Fig. 2. From Fig. 2 (a, b) one can clearly see that the particle size of pristine NVP is in the range of 50e150 nm. Further, the corresponding high magnification image shows the presence of clear lattice fringes with a d spacing value of 0.28 nm, which corresponds to the (116) lattice

Fig. 2. HRTEM images of (aeb) pristine NVP; (c) SAED pattern of pristine NVP, (dee) NVP/HHC-10 (in-situ); (f) SAED pattern of NVP/HHC-10 (in-situ), (geh) NVP/HHC-10 (ex-situ); (i) SAED pattern of NVP/HHC-10 (ex-situ) and (jek) NVP/HHC-20 (ex-situ); (l) SAED pattern of NVP/HHC-20 (ex-situ).

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plane of NVP. The obtained results are in accordance with the XRD results (Fig. 1). The SAED pattern (Fig. 2c) of select particle is shown in the Inset of Fig. 2b, which reveals the presence of highly ordered single crystalline nature of NVP. On the other hand, NVP/ HHC composites show the uniform distribution of NVP nanoparticles (50e100 nm) in the randomly oriented graphene sheet like structure of HHC, which is clearly seen from Fig. 2 (d, g, j). The complete wrapping of NVP nanoparticles on the surface of HHC sheets is obviously seen from the high magnification HRTEM images (Fig. 2 e, h, k) and the calculated d spacing value (0.28 nm corresponding to the (116) lattice plane of rhombohedral NVP) from the observed fringes is in good agreement with the XRD results [4,22,40e43]. The SAED patterns recorded for all the samples, shown as the inset of respective images clearly reveal the single crystalline nature of NVP [43,45]. Such a complete wrapping of NVP nanoparticles by randomly oriented and interconnected mesoporous HHC sheets allows easy and facile penetration of electrolyte and avoids the agglomeration of nanoparticles during electrochemical reaction. Moreover, the mesoporous nature of HHC can reduce the path length of Naþ diffusion, thereby providing scope for the improved sodium ion transport kinetics facilitated appreciable specific capacity, particularly applicable at high charge-discharge rates. In addition, HHC can offer fascinating advantages by trapping multiple NVP nanoparticles, which results in improved electronic conductivity and faster Na-ion migration. In particular, graphene like HHC sheets will be expected to mitigate the huge volume change that occurs during the intercalation/deintercalation of sodium ions.

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5.2. Electrochemical characterization Fig. 3 shows the CV behavior (five cycles) of pristine NVP and NVP/HHC composite anodes recorded between 0.01 and 3.0 V vs. Naþ/Na at a scan rate of 0.05 mV s1. In the first cycle, all the anodes display two reversible redox pairs at 1.5/1.7 V and at 0.2/0.4 V. Herein, the reversible redox pairs could be associated with the reversible reduction/oxidation of V3þ/V2þ and V2þ/V1þ, corresponding to the step-wise insertion of two sodium ions, respectively (as indicated in equation (1)). The redox potential is comparable with the previous reports on NVP [44,45]. More interestingly, this observation is consistent with the voltage plateau found in the charge-discharge behavior of pristine NVP and NVP/ HHC composite anodes, examined individually (Fig. 4). 0:013:0 V

Na3 V2 ðPO4 Þ3 þ 2Naþ þ 2e ƒƒƒƒƒ ƒƒƒƒƒ ƒ! ƒ Na5 V2 ðPO4 Þ3

(1)

The smaller potential difference observed between the oxidationereduction peaks of NVP/HHC composite anodes compared with that of pure NVP could be attributed to the variation of internal resistance of the cell as a function of added carbon to form the composite. Similarly, couple of well-defined reductioneoxidation peaks corresponding to the presence of V3þ/V2þ and V2þ/V1þ redox pairs, substantiating the extraction/insertion process of Naþ in the NASICON-type structured NVP is observed for all the NVP/HHC composite anodes, but not for pristine NVP anode, which is quite interesting. The peak current of NVP/HHC composite anodes is about ~0.14 mA, which is much higher than that observed

Fig. 3. CV behavior of (a) pristine NVP, (b) NVP/HHC-10 (in-situ), (c) NVP/HHC-10 (ex-situ) and (d) NVP/HHC-20 (ex-situ) anode.

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for pure NVP anode (0.08 mA). Since the Naþ diffusion coefficient is proportional to the square of the peak current (ip2) in the CV profile, the currently observed higher peak current behavior endorses the possibility of realizing higher Naþ diffusion coefficient and consequently much improved electrochemical performance of chosen NVP/HHC composite anode materials has been realized. Presence of well resolved and intense CV peaks of NVP/HHC composite anodes could be attributed to the functionality of HHC. Moreover, the perfect overlap of CV curves upon subsequent cycles suggests that NVP/HHC composite anodes are more stable than pristine NVP in non-aqueous electrolytes, which is also encouraging. Despite these reversible peaks, another irreversible peak is observed at 0.49 V in the first cathodic scan, which in fact corresponds to the decomposition of electrolyte and the associated formation of SEI layer over the electrode surface [44,45]. However, the subsequently observed identical cyclic voltammograms upon extended cycles (after the first cycle), could be attributed to the stable SEI formation and cycling stability of the electrodes. It is important to mention here that the minor redox pairs found at 0.05/0.06 V has been observed for all the anodes including pristine NVP, which could be attributed generally to the sodiation/desodiation of the carbon coating layer [44,45]. Hence, the effect of added carbon and the residual carbon upon CV behavior pertinent respectively to NVP/HHC composites and pristine NVP could be understood. Similar behavior has been observed for the carbon based materials deployed in SIBs [44,45], which is encouraging.

Voltage profile of pristine NVP and NVP/HHC composite anodes recorded at a current density of 50 mA g1 are shown in Fig. 4. There are three flat plateaus, located at 1.5, 0.5 and 0.2 V, respectively, which are consistent with the redox peaks in the CV curves. The lengthy slope in the initial sodiation curve is caused by the SEI formation, corresponding to the cathodic peak at 0.5 V in the CV curve. The plateau region at 1.5 V is associated with the insertion of the first Na ion. Another plateau at 0.2 V corresponds to the insertion of second Na ion. These results suggest that two Na ions can be inserted into the rhombohedral NVP [44,45]. The plateau at 0.05 V is probably attributed to the insertion of sodium ions into the coated carbon layer [45]. The initial discharge capacity value of pristine NVP, NVP/HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/ HHC-20 (ex-situ) anodes are 203, 287, 208, 373 mAh g1, which are higher than the theoretical capacity of NVP and such an observation is due to the permissible side reactions involving electrolyte decomposition and SEI formation. The first charge capacity value of pristine NVP, NVP/HHC-10 (insitu), NVP/HHC-10 (ex-situ) and NVP/HHC-20 (ex-situ) anodes are 80, 132, 83, 158 mAh g1, indicating a Coulombic efficiency of 39, 46, 40 and 42%. After the first cycle, the Coulombic efficiency increases substantially to 86, 98, 98 and 98% for all the anodes, indicating the formation of stable SEI on the electrode surface. Moreover, the NVP/ HHC composite anodes show higher Coulombic efficiency value than the pristine NVP anode and the same could be correlated to the addition of HHC as a composite additive. In particular, NVP/ HHC-10 (in-situ) and NVP/HHC-20 (ex-situ) anodes exhibit a

Fig. 4. Charge/discharge profile of (a) pristine NVP, (b) NVP/HHC-10 (in-situ), (c) NVP/HHC-10 (ex-situ) and (d) NVP/HHC-20 (ex-situ) anode.

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steady state and reversible electrochemical performance with higher Coulombic efficiency value (98 and 98%) than other anodes (86 and 96%). Hence, it is believed that the multiple appealing features of externally or internally added HHC will boost the electrochemical performance of pristine NVP, especially when used as a composite additive. The cycling performance of pristine NVP and NVP/HHC composite anodes at a current density of 50 mA g1 is displayed in Fig. 5. After 100 cycles, the capacity value obtained for pristine NVP, NVP/HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/HHC-20 (exsitu) anodes is 30, 100, 53 and 126 mAh g1 with a capacity retention of 55, 80, 90 and 95%, respectively. From this observation, it is understood that NVP/HHC-10 (in-situ) and NVP/HHC-20 (exsitu) anodes exhibit better cycling performance compared with any other reported values of NVP anode and with those of pristine NVP and NVP/HHC-10 (ex-situ) anodes of the present study (Table S1). Such a high capacity observed for NVP/HHC-10 (in-situ) and NVP/ HHC-20 (ex-situ) anodes could be attributed to the addition of HHC as composite additive that leads to significant enhancement in the dynamic electrochemical stability of the sodium storage. On the other hand, pristine NVP that shows poor electrochemical performance indicates the essential requirement of carbon coating to achieve excellent electrochemical performance, due to electronic conductivity related intricacies pertinent to the pristine NVP anode [44,45]. Similarly, NVP/HHC-10 (ex-situ) anode is also not showing promising electrochemical performance, since the externally added 10 wt% of HHC is not felt to be sufficient enough to provide a continuous and conducting network for NVP nanoparticles required especially upon prolonged cycling. Because, ex-situ carbon addition, known for heterogeneous distribution of carbon is found to suffer from inadequate concentration of carbon, required to

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ensure a continuous and conducting electron pathway, which could be made possible with the same concentration of carbon, especially when added along with the reactants to obtain the in-situ composite. Hence, the advantage of in-situ carbon coating over ex-situ strategy and the possibility of exploiting minimum amount of additive required to obtain the desired characteristics through in-situ composite formation could be understood. The same is better understood from the electrochemical behavior of NVP/HHC-10 (insitu) and NVP/HHC-20 (ex-situ) anodes, wherein complete wrapping of NVP nanoparticles is ensured by the 10 wt% in-situ addition and 20 wt% ex-situ addition of carbon to realize the advantages of N and S doped graphene sheet like HHC in improving the electrochemical behavior of native NVP. In particular, the added HHC increases the conductivity of the composite anodes and acts as a buffering matrix by offering the required space to admit volume changes and to prevent the pulverization, during sodiation/desodiation. Further, the presence of mesopores in HHC offers the possibility of faster sodium ion transfer, resulting in the reduced diffusion path lengths of Naþ/e and favors the electrolyte penetration to realize better electrochemical performance with NVP/ HHC composite of select category. Further, with a view to investigate upon the rate capability behavior of pristine NVP and NVP/HHC composite anodes, all the anodes were subjected individually to charge-discharge cycling studies under the influence of different current rates, wherein each current rate was made applicable to five cycles and the corresponding capacity values observed are depicted in Fig. 6 and Fig. S6. The outstanding rate capability behavior of NVP/HHC-20 (ex-situ) anode is demonstrated by the observed specific discharge capacity values of 110, 100, 80, 50 and 30 mAh g1 at current densities such as 100, 200, 500, 1000 and 2000 mA g1 respectively. Even at a high

Fig. 5. Cycling performance of (a) pristine NVP, (b) NVP/HHC-10 (in-situ), (c) NVP/HHC-10 (ex-situ) and (d) NVP/HHC-20 (ex-situ) anodes at a current density of 50 mA g1.

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current density of 2 A g1 (17 C), NVP/HHC-20 (ex-situ) anode delivers a reversible capacity of 30 mAh g1 for 500 cycles (Fig. S7), which is superior than any other reported NVP anodes (Table S1). After cycling at various high current densities up to 2000 mA g1 level, the current density was brought back to 100 mA g1 and the specific capacity that could be recovered was found to be 100 mAh g1, implying the highly stable cycling performance and the excellent reversibility of NVP/HHC-20 (ex-situ) anode. As expected, the discharge capacity decreases drastically with the increasing current density for pristine NVP and NVP/HHC-10 (ex-situ) anodes. Under the influence of moderate discharge conditions such as 50, 100, 200, 500 and 1000 mA g1, the rate capability behavior is found to be acceptable for NVP/HHC-10 (in-situ) anode. i.e., specific discharge capacity values of 100, 80, 70, 60 and 40 mAh g1 are observed at a current density of 50, 100, 200, 500 and 1000 mA g1, respectively. A reversible capacity of 100 mAh g1 has been obtained, when the cell was switched back to the initial current density condition (100 mA g1). From the observed results, it is understood that the unique nanostructure of NVP and its complete wrapping with the randomly oriented graphene sheet like structure of HHC effectively shortens the sodium diffusion length, providing faster mass transfer with the effective alleviation of severe volume change effects. Apart from the presence of reasonable concentration of nitrogen (5.11%) and sulfur (1.53%) along with the desired ratio of nitrogen to sulfur content, the currently investigated HHC as a compositing additive possesses a high percentage of active pyridinic N (37.55%) and pyrrolic N (62.45%), which are

responsible for the homogeneous distribution and effective participation of active sites. In short, the presence of pyridinic-N species in NVP/HHC composites facilitates the exposure of planar edges or defect sites in carbon materials, while pyrrolic N and graphitic-N will enable the surface adsorption and increased electronic conductivity advantages of the carbon matrix respectively. As a result, nitrogen-doped carbon would be electrochemically more active. Further, high electronegativity of nitrogen (3.04) and sulfur (2.58) than carbon (2.55), makes NVP/HHC composite anodes to assume a non-electroneutral state with many charged sites, which in turn are favorable for the reversible transport of sodium ions. It is very important to mention here that the high Naþ diffusion kinetics of NVP/HHC-20 (ex-situ) is attributed to the larger active surface area of the composite (120.27 m2 g1), presence of meso/micropores rich electroactive matrix, desirable for efficient electrochemical reaction and the fast electrolyte percolation aided by porosity benefits [49,56,59]. The overall synergistic effect of the above mentioned advantages of NVP/HHC composite anodes (NVP/HHC-10 (in-situ) and NVP/HHC-20 (ex-situ)) thus favors the long term cycling behavior and significantly enhances the sodium storage performance at high rates, say @ 2 A g1 condition. Further, with a view to understand the role of HHC in improving the charge transfer kinetics of sodium ion, EIS analysis was carried out for pristine NVP and NVP/HHC composite samples. EIS spectra of as fabricated cells and the cells after completing 100 charge/ discharge cycles containing NVP and NVP/HHC composites as anode are shown in Fig. S8. The semicircle at the high to medium

Fig. 6. Rate capability behavior of (a) pristine NVP, (b) NVP/HHC-10 (in-situ), (c) NVP/HHC-10 (ex-situ) and (d) NVP/HHC-20 (ex-situ) anode.

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Fig. 7. Post cycling XRD pattern of pristine NVP and NVP/HHC composites after 100 cycles at 50 mA g1.

frequency region shows the kinetically controlled process and the sloped line in the low frequency region represents the Warburg impedance related to the sodium-ion diffusion in NVP [44,45]. The Nyquist plots show that the charge transfer resistance (Rct) of freshly assembled pristine NVP and NVP/HHC-10 (in-situ), NVP/

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HHC-10 (ex-situ) and NVP/HHC-20 (ex-situ) composite anodes are 120, 200, 150 and 200 U, respectively. The NVP/HHC-10 (in-situ) (195 U) and NVP/HHC-20 (ex-situ) (200 U) composite anodes exhibit low charge transfer resistance and high sodium-ion diffusion even after 100 cycles compared with those of other anodes, such as pristine NVP (230 U) and NVP/HHC-10 (ex-situ) (290 U). This result once again confirms that the interconnected HHC layers increase the conductivity of the composite electrode and offer the required long term cycle life benefit to SIBs. Moreover, the porous structure of HHC is believed to play a vital role in facilitating the electrolyte penetration into the composite along with the reduction of transport path of Naþ through graphene sheet like carbon towards NVP nanoparticles. To support and to understand the factors governing the excellent and long term electrochemical performance of NVP/HHC composites, series of ex-situ characterization studies related to the electrode material after completing 100 cycles at 50 mA g1 were carried out. Fig. 7 shows the post cycling XRD pattern obtained for pristine NVP, NVP/HHC-10 (in-situ), NVP/HHC-10 (ex-situ) and NVP/ HHC-20 (ex-situ) samples, individually. All the diffractions peaks exactly match with the R3c rhombohedral NVP phase without any impurities, indicating the desired and perfect maintenance of structural stability (Fig. S9). The complete wrapping of NVP nanoparticles by randomly oriented graphene sheet like HHC can be clearly observed from the post cycling SEM, TEM and HRTEM images (Fig. S10, S11 and Fig. 8) of NVP/HHC composite electrode materials. In particular, lattice fringes of different planes can be clearly indexed, demonstrating the maintenance of highly ordered single crystalline nature of NVP even after completing 100 charge/discharge cycles [42,45,46]. More interestingly, NVP nanoparticles that are uniformly distributed in the HHC sheets in the NVP/HHC-10 (in-situ) and NVP/HHC-20 (exsitu) electrode materials clearly explain the superior electrochemical performance over other anodes. On the other hand, from the observed post cycling SEM, TEM and HRTEM images (Fig. S10, S11 and Fig. 8), the increased size of pristine NVP particles due to

Fig. 8. Post cycling HRTEM images of (aeb) pristine NVP; (c) SAED pattern of pristine NVP, (dee) NVP/HHC-10 (in-situ); (f) SAED pattern of NVP/HHC-10 (in-situ), (geh) NVP/HHC10 (ex-situ); (i) SAED pattern of NVP/HHC-10 (ex-situ) and (jek) NVP/HHC-20 (ex-situ); (l) SAED pattern of NVP/HHC-20 (ex-situ), after 100 cycles.

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cycling is evident which in turn explains the reason for the poor electrochemical performance of pristine NVP anode. 6. Conclusions In summary, we have proposed a simple, facile, convenient, effective and scalable composite formulation of NVP/HHC composite anode, in which NVP nanoparticles are wrapped with N and S doped randomly oriented graphene sheet like carbon derived from human hair. The sodium storage performance of NVP/HHC anode with different carbon contents has been investigated to understand the optimum amount of carbon and the mode of carbon addition desired to realize appreciable electrochemical performance with respect to NVP/HHC composite anodes. It was found that NVP/HHC10 (in-situ) (HHC is a cheap and eco-friendly composite additive, obtained from bio-waste (human hair)) and NVP/HHC-20 (ex-situ) anodes display excellent electrochemical performance for about 500 cycles with appreciable Coulombic efficiency (98%), acceptable storage capacity (125 mAh g1) and high rate capability (2000 mA g1). Stable crystal structure of NVP and complete wrapping of NVP nanoparticles by HHC sheets in NVP/HHC composite anodes found even after the completion of 100 cycles as revealed by ex-situ XRD, SEM, TEM and HRTEM analyses are in strong support of the involvement of reversible intercalation/ deintercalation mechanism. The HHC wrapping design offers intimate contact between the NVP nanoparticles, suppresses the particle agglomeration and mitigates the volume changes experienced by NVP during sodiation and desodiation. Further, dual hetero atom doping and presence of mesopores in HHC facilitate the fast charge transfer across the electrode/electrolyte interface, which improves the rate capability at high rates. The newer/novel composite formulation could be extended to other materials with poor electronic conductivity for energy storage in upcoming battery assemblies. Acknowledgements One of the authors (Saravanan Karuppiah) is grateful to CSIR for the CSIR-SRF grant. Financial support from Council of Scientific and Industrial Research (CSIR) through MULTIFUN program is gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.11.095. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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