electrospinning as anode material for Li- ion batteries

electrospinning as anode material for Li- ion batteries

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Materials Today Energy 4 (2017) 14e24

Contents lists available at ScienceDirect

Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/

Nano-grained SnO2/Li4Ti5O12 composite hollow fibers via sol-gel/ electrospinning as anode material for Li- ion batteries Anulekha K. Haridas a, b, 1, Chandra S. Sharma b, *, Neha Y. Hebalkar a, Tata N. Rao a, ** a b

Centre for Nanomaterials, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, Telangana, India Department of Chemical Engineering, Indian Institute of Technology, Hyderabad, Kandi, Telangana, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 October 2016 Received in revised form 16 December 2016 Accepted 8 January 2017

The high capacity of SnO2 (tin oxide) and high rate capability of Li4Ti5O12 (lithium titanate, LTO) were pooled together for engineering a composite Li ion anode material in hollow fiber edifice by sol-gel/ electrospinning. The electrospun porous precursor composite hollow fibers (CHFs) were heat treated either in air (SnO2/LTOA) or argon (SnO2/LTOAr) atmosphere to control grain size, porosity and presence of Ti3þ content. The morphological study performed using Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy revealed smaller grain size (5e10 nm) for SnO2/LTOAr CHFs. Further, X-Ray Diffraction and X- Ray Photoelectron Spectroscopy studies illustrated a significant variation in the crystallinity and the elemental oxidation states in these CHFs respectively. BrunauerEmmett-Teller measurements exposed the presence of high surface area and pore volume in SnO2/ LTOAr CHFs. Further, the half-cell galvanostatic charge-discharge performances of SnO2/LTOAr CHFs at 1 C rate revealed a stable specific capacity of 300 mA h/g for 110 cycles with 90% capacity retention. The stable and high capacity of SnO2/LTOAr CHFs were corroborated to the presence of smaller grain size, high porosity and conductive Ti3þ providing faster lithium ion diffusion when compared to SnO2/LTOA CHFs. Electrochemical Impedance Spectroscopy study confirmed low impedances in SnO2/LTOAr CHFs due to low charge transfer and electrolyte resistances. Moreover, Li ion full-cell study performed using LiFePO4 (LFP) cathode (3 V), delivered a specific capacity of 230 mAh/g at 0.1 C rates. The excellent electrochemical performance of SnO2/LTOAr CHFs in both half-cell and full-cell modes illustrated the significance of sol-gel/electrospinning in synthesizing high performance Lithium ion batteries in a cost effective and scalable way. © 2017 Elsevier Ltd. All rights reserved.

Keywords: SnO2/Li4Ti5O12 Nano grains Hollow fibers Anode material High rate Lithium ion battery

1. Introduction Design and synthesis of high performance Li ion battery electrode materials by scalable processes are on high demand as LIBs are being used in various electronic applications and electric vehicles [1,2]. Specific capacity, cell voltage and rate capability are the three major parameters that have to be taken into consideration while engineering an electrode material. The present lithium ion battery electrode materials with high capacities and energy densities such as Sn/SnO2 and Si/SiO2 were affected by huge volume expansion during lithium insertion (charging) as a result of the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.S. Sharma), [email protected] (T.N. Rao). 1 Present address: Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas, 77005, USA. http://dx.doi.org/10.1016/j.mtener.2017.01.002 2468-6069/© 2017 Elsevier Ltd. All rights reserved.

alloying reaction with lithium [3e9]. During cell discharge, dealloying process causes disintegration of the crystal structure and result in poor contact between electrode material and current collector ensuing rapid capacity fading. Moreover, mechanical disintegration during de-alloying generates fractured metal surfaces. These fractured metal surfaces reacts with electrolyte and lead to loss of active metal from the electrode. Despite of these disadvantages, Sn/SnO2 is still considered as a significant anode material as they can accommodate 4.4 Li providing a theoretical capacity of 782 mAh/g with maximum volume expansion of 300% [3,10,11]. Several methods were employed to tackle the volume expansion and mechanical disintegration of Sn based electrode materials. In these methods, porosity and minimizing particle size control by nanostructuring are mainly being explored [3,5,10,11]. Meanwhile, nanostructured Sn and SnO2 in the form of hollow morphologies such as tubes and spheres were attempted by researchers and were

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found to be effective in improving the electrochemical performances when compared to the solid nanostructures [12]. However, the capacity fade still remained an unsolved issue as the crystal structure disintegration of Sn/SnO2 cannot be arrested by these designs. In order to address the capacity fade, nanocomposites of these materials were prepared with carbon nanotubes, graphene or mesoporous carbon to create porosity, electrode structure integrity and also for improving electronic conductivity [13e19]. Even though the performances of these Sn based carbon nanocomposites showed excellent capacity values, the consistency in composite preparation, cost effective scaling up methods, as well as varying electrochemical performances are serious issues in a commercialization point of view. Hence research endeavors like in situ synthesis of SnO2 composites with other metal oxides such as TiO2 (SnO2/TiO2) [20e22] and LTO (SnO2/LTO) [23e29] were explored. The presence of second oxide phase in these composite systems minimized the volume expansion and pulverization of Sn by forming smaller sized particles during fabrication of the composite [20e29]. However, these composite structures failed to provide high rate capability and cycle stability due to their low conductivity as well as low porosity caused diffusion limitation. Thus an ideal SnO2 based anode that is expected to minimize the growth of Sn particles can be a highly conductive and porous nanocomposite comprising of SnO2 and another oxide. In order to address diffusion limitation and the low conductivity in the above mentioned SnO2/oxide composites, we have designed a new anode material in the form of SnO2/LTO composite hollow fibers (CHFs) with considerable Ti3þ in the lattice. This unique structure with high porosity can ease Li ion diffusion in the electrode. As the cycle stability and high rate capability of LTO is well known due to its zero strain nature (~0.2% volume expansion) during lithium ion insertion and desertion [30e35], an improved structural integrity is expected for the SnO2/LTO composite anode. LTO is also a high voltage anode material (1.55 V versus Li) that exhibits theoretical capacities 175 mAh/g in the voltage range 1e2.5 V and ~300 mAh/g when cycled from 0 to 2.5 V [30e35]. This is the first attempt in the literature that brings together SnO2 and LTO in the form of composite hollow fibers with controlled grain size and composite phase distribution. Sol-gel/electrospinning method was employed to fabricate SnO2/LTO CHFs. The process of electrospinning is a well-known method that utilizes high voltages (in the order of kilo volts) for fabricating nanofibers [36,37]. Meanwhile, sol-gel process can aid in the uniform distribution of both oxide phases for obtaining high purity nanocomposite. Combining sol-gel process and electrospinning is an easy, simple, scalable and cost effective method to fabricate nanostructures of various shapes such as rods, fibers, donuts etc. as discussed in our previous works [31,32,38]. By considering the above discussion in mind; we have fabricated SnO2/LTO CHF by a two-step process. In the first step, SnO2/LTO precursor CHFs is fabricated by sol-gel/electrospinning. In the second step, the so formed precursor hollow fibers were heat treated in two different atmospheres viz. air or argon. The two different atmospheric heat treatments were meant to have variances in the composite characteristics viz. particle size, porosity and absence/presence of Ti3þ. The synthesis, characterization and the electrochemical performances of SnO2/LTO CHFs prepared in both atmospheres are discussed in detail. 2. Experimental 2.1. Materials Titanium (IV) isopropoxide (TIP) 97% and Tin (IV) isopropoxide (SnIP) 10 wt/v in isopropanol were purchased from Alfa Aesar,


India. Ethanol (99% pure), Lithium acetate di hydrate, Tin 2 ethyl hexanoate (T-2EH) and polyethylene oxide (PEO) with molecular weight 90, 0000 g/mol are obtained from sigma Aldrich, India. All the chemicals were used as such without any modification. 2.2. Preparation of SnO2/LTO CHFs by sol-gel/electrospinning Initially SnO2 sol and LTO sol were prepared separately and mixed in 1:1 ratio to make the precursor sol for SnO2/LTO CHF. For preparing SnO2 sol, 1 ml acetyl acetone was added to 2 ml ethanol and mixed thoroughly. SnIP was added to the above solution and was stirred for 30 min followed by drop wise addition of 500 ml water. Then the above sol was aged for 6 h. Later T-2 EH was added to the aged sol, stirred for 15 min and kept for 7 days ageing. The concentration Sn precursors in the SnO2 sol were 33% (v/v) SnIP and 13% (v/v) T-2EH. LTO sol was prepared as reported in our earlier publication [32] using TIP and lithium acetate di hydrate in Li: Ti ratio 4: 5. Then the SnO2 sol and LTO sol were mixed in 1:1 ratio by stirring for 1 h, and electrospun into SnO2/LTO precursor CHF. As a control study, SnO2/PEO precursor fibers were also prepared by electrospinning from a blend of SnO2 sol and 20 wt % PEO solutions (in water). E-spin Nano machine, India was used for electrospinning process using 10 ml syringes fitted with 21 gauge needles. A voltage of 25 kV and 10 cm distance between the electrodes were maintained during electrospinning process and an aluminium foil was used for collecting the electrospun precursor fibers. A part of these SnO2/LTO precursor CHFs were then calcined in air at various temperatures 550, 650 and 750  C for 1 h to obtain SnO2/LTOA CHFs. Another part of SnO2/LTO precursor CHFs were annealed in argon atmosphere at 550 and 650  C for 4 h to obtain SnO2/LTOAr CHF. Similarly, bare SnO2 fibers and SnO2/C fibers were prepared by heat treating the electrospun SnO2/PEO precursor fibers in air and argon atmosphere respectively. A schematic is provided (Scheme 1) for illustrating the synthesis of hollow fibers in air and argon atmospheres and were named hereafter as SnO2/ LTOA and SnO2/LTOAr CHF respectively. 2.3. Material characterization Morphology of SnO2/LTOA and SnO2/LTOAr CHFs was analyzed using Field Emission Scanning Electron Microscopy (FESEM, Hitachi model- S4300SE/N). The hollow nature and particle size in the fibers were further studied by Transmission electron microscopy (TEM, Tecnai G2 TF20 TWIN) at an operating voltage of 200 kV. XRay diffraction (XRD, Bruker D8 with Cu Ka radiation) was used for understanding the crystallinity and phase formation in the fibers from 10 to 80 angle. The diffraction patterns were then analyzed using Match software with JCPDS data cards for confirming the phase formation. The elemental composition and their oxidation states in the CHFs were also studied using X-ray Photo electron spectroscopy (XPS, Omicron with Al Ka radiation). The surface area, pore size and pore volume of the samples, were studied using surface area and porosity analyzer (Micrometrics ASAP 2020) using BET and BJH method. 2.4. Electrode preparation and electrochemical characterizations Electrodes of SnO2, SnO2/C fibers and SnO2/LTOA and SnO2/LTOAr CHF were made by preparing slurries of the respective fibers (80 wt %) with conductive carbon (10 wt %) and PVDF (10 wt %) in Nmethyl pyrrolidone. The prepared slurries were coated on battery grade Cu foil obtained from MTI Corporation, United States and was dried in vacuum for 18 h. Circular discs of 12 mm diameter were cut from the dried Cu foils and were assembled into coin cells inside glove box in argon atmosphere. Whatman micro glass fibers were


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Scheme 1. Schematic depicting the synthesis process of SnO2/LTOA and SnO2/LTOAr CHFs by electrospinning and post heat treatments.

Fig. 1. FESEM images of as electrospun precursor hollow fibers (A and B), air treated SnO2/LTOA CHFs (C) and argon treated SnO2/LTOAr CHFs (D). XRD pattern of SnO2/LTOA CHFs (750  C) and SnO2/LTOAr CHFs (650  C) are shown in E.

used as separator and Li foil was used as the counter electrode. 1 M LiPF6 dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) was used as electrolyte (EC: DMC ¼ 1:1). Electrochemical half-cell performance of bare SnO2 fibers, SnO2/C fibers, SnO2/LTOA CHFs and SnO2/LTOAr CHFs were studied using Arbin Instruments from 0.02 V to 1.5 V at various charging rates. Cyclic voltammetry studies were carried out at a scan rate of 0.02 mV/s for 6 cycles. Further, electrochemical impedance measurements were conducted at frequency range of 0.01e100 MHz using electrochemical impedance analyzer Solartron, at amplitude of 10 mV. For demonstrating the working of SnO2/LTO CHFs in a complete Li ion cell, SnO2/LTO CHFseLFP full cell was fabricated with commercially available LiFePO4 (LFP) as cathode. The cathode was prepared on Al foil with 80 wt % active material, 10 wt % PVDF and 10 wt % conductive carbon. Full-cell charge discharge study was performed in the voltage range of 1.5e3.5 V.

3. Results and discussion 3.1. Formation of SnO2/LTO CHFs: morphology and phase analysis SnO2 and LTO sols were mixed in 1:1 M ratio and aged for various time intervals to obtain SnO2/LTO precursor spinning solutions. Prior to electrospinning, the viscosities of spinning solutions were measured as 14 ± 2 mPa s, 40 ± 5 mPa s and 70 ± 10 mPa s respectively for 6 h, 48 h and 7 days of ageing.

Electrospun sols after 6 h, 48 h and 7 days of aging yielded particles, dimpled spheres and hollow fibers respectively. A detailed description about formation of various structures on electrospinning is provided in the supplementary information (Fig. S1 and S2). Meanwhile, SnO2 sol and PEO solution (in 1:1 vol ratio) were blended and electrospun into SnO2/PEO fibers. SEM image of electrospun SnO2/PEO precursor fibers (control study) was observed with fiber diameters in 700 nm- 1 mm range (Fig. S3A). Fig. 1A and B shows the FESEM images of as-spun SnO2/LTO precursor CHF with average diameter in the range of 1e2 mm. Here, the formation of fiber morphology during electrospinning was correlated to the sol ageing period which in turn was related to the viscosity of the spinning solution. As the gelation time was increased, viscosity of spinning solution was also observed as increasing within seven days yielding fiber morphology. Similar results were observed in our previous studies with LTO sol which resulted in particles, donut structures and rod shaped structures as a function of solution viscosity [32]. The formation of porous hollow fiber morphology can be attributed to the differences in volatilities of solvent mixtures (ethanol and isopropanol) in the spinning solution [39]. During electrospinning process, SnO2/LTO precursor materials in the spinning fiber are driven in the radial direction. The more volatile solvent ethanol evaporates faster creating a concentration difference of ethanol and isopropanol in the spinning fiber. As the volatility of isopropanol is also in the similar range of ethanol, soon after the evaporation of later,

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Fig. 2. TEM images of SnO2/LTOACHFs (A1, B1) and SnO2/LTOAr CHFs (A2, B2) showing the hollow nature and the presence of small grains.

Fig. 3. A in the Fig. indicates the XPS surface spectrum of SnO2/LTOA and SnO2/LTOAr CHFs. Sn, Ti, O, Li and C de-convoluted peaks in the CHFs are depicted in B, C, D, E and F of the figure.

isopropanol also escapes from the structure creating porous hollow fiber morphology. In order to obtain SnO2 and LTO phases in the hollow fibers, the precursor CHFs were calcined in air and argon atmospheres at various temperatures. FESEM images of SnO2/LTOA and SnO2/LTOAr CHFs obtained after calcination in air (750  C) and argon (650  C) atmospheres respectively were displayed in Fig. 1C and D. The air heat treated SnO2/LTOA hollow fibers at 750  C were perceived with diameters in the range of 1e2 mm. Meanwhile, electrospun SnO2/ PEO fibers were heat treated in air and argon atmospheres to obtain SnO2 fibers and SnO2/C fibers respectively. SEM images of SnO2

fibers and SnO2/C fibers were provided in Fig. S3B and S3C respectively. As the temperature of calcination was increased from 550 to 750  C, crystallinity in the SnO2/LTOA CHFs was perceived as increasing, as shown by XRD pattern in Fig. S4A. The presence of both SnO2 (cassiterite) and LTO (spinel) phases in the CHFs were confirmed by matching with JCPDS data cards 41e1445, (space group P42/mnm) and 04-016-2284 (space group Fd3m) respectively. The planes (110), (101), (200) (211) and (301) are matched with cassiterite phase of SnO2 whereas (111), (311), (400), (333) and (440) are matched with spinel phase of LTO. XRD pattern of bare SnO2 fibers obtained by heat treating electrospun SnO2/PEO fibers


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Table 1 Summary of BET results in SnO2/LTOA and SnO2/LTOAr CHFs. Sl. No

Hollow fibers

BET surface area (m2/g)

Pore size (nm)

Pore volume (cm3/g)

1 2


102.40 12.55

4.64 28.40

0.11 0.09

at 750  C are also provided in Fig. S4A. Similarly, argon heat treatment was conducted from 550 to 650  C for 4 h. Here, the argon heat treatments were limited up to 650  C due to the chance of losing Sn at higher temperatures. XRD patterns of the argon treated samples at 550 and 650  C are shown in Fig. S4B. XRD pattern of SnO2/C fibers obtained by heat treating electrospun SnO2/PEO fibers at 650  C in argon are also provided in Fig. S3B. SnO2/LTOAr hollow fibers at 650  C were observed with broad peaks of SnO2 and LTO when compared to the air heat treated SnO2/LTOA CHFs at the same temperatures. XRD pattern of SnO2/LTOA (750  C in air) and SnO2/LTOAr (650  C in argon) were provided in Fig. 1E and were chosen for further characterizations and electrochemical studies as they were of higher crystallinity in the respective atmospheric heat treatments. TEM analysis of the SnO2/LTOA and SnO2/LTOAr CHFs were given in Fig. 2A1 and 2A2 respectively. A bright interior and dark exterior contrast difference was observed in both CHFs confirming the hollow morphology. Further, the high magnification TEM images (Fig. 2B1 and B2) revealed porous nature of CHFs with the presence of small sized grains. The grain size of SnO2/LTOA (Fig. 2B1) was about 10e30 nm, whereas SnO2/LTOAr was observed with grains ranging from 5 to 15 nm in diameter with a small fraction of bigger grains (Fig. 2B2). The smaller grain sizes observed in SnO2/LTOAr can be attributed to the controlled heat treatment which reduced the grain growth in the absence of oxygen that would rather happen in the air atmosphere. The selected area diffraction (SAED) pattern of SnO2/LTOA CHFs and SnO2/LTOAr CHFs were observed as dark and bright circular bands. The bright diffraction pattern in SnO2/LTOA CHFs indicated their highly crystalline nature, whereas the diffused pattern in SnO2/LTOAr CHFs indicated the presence of smaller crystallites in the amorphous matrix (Fig. S5 A and B).

3.2. XPS study of SnO2/LTO CHFs

Fig. 4. Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) of SnO2/LTOA and SnO2/LTOAr CHFs.

In order to understand the oxidation states of Sn and Ti in SnO2/ LTOA and SnO2/LTOAr CHFs XPS study was performed in the binding energy range of 0e800 eV. The surface spectra of CHFs (Fig. 3A) revealed the presence of tin, oxygen, titanium and carbon. The composition of both CHFs are represented in Table S1. The

Fig. 5. Cyclic Voltammetry studies of SnO2/LTOA (A) and SnO2/LTOAr CHFs (B) at a scan rate of 0.02 mV/s.

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elemental ratio of Sn and Ti in SnO2/LTOA and SnO2/LTOAr hollow fibers were observed as 0.8:1 and 0.6:1 respectively. The Sn 3d5/2 peaks in SnO2/LTOA and SnO2/LTOAr hollow fibers were observed at binding energy 486.6 eV and 486.75 eV respectively (Fig. 3B). This indicated the presence of Sn4þ in the CHFs with Sn bonded to oxygen as in SnO2 [40]. Further, the Ti 2p3/2 peak at 458.7 eV in SnO2/LTOA CHFs was co-related to the presence of Ti4þ which had Ti bound to O as in LTO [36,37] (Ti 2p3/2, Ti 2p1/ 2 ¼ 458.74, 464.56 eV). Whereas the Ti 2p3/2 peak in SnO2/LTOAr hollow fibers was observed at 456.1 eV which was seen shifted to lower binding energy by 2.6 eV indicating the presence of Ti3þ oxidation state (Fig. 3C). The de-convolution of O 1s peak (Fig. 3D) in SnO2/LTOA CHFs have shown two peaks at 529.52 eV and 530.16 eV pertaining to OTi and O- Sn bonds respectively as observed in LTO and SnO2 [40e42]. Lithium was detected in both the CHFs at 55 eV that matched the Liþ1 oxidation state (Fig. 3E). The presence of carbon was detected in both samples, relating the peaks at 284.6 and 285 eV to C- C and C¼ C respectively [43,44]. SnO2/LTOA CHFs were observed with peaks having higher intensity and slight binding energy shift towards lower value when (Fig. 3F) compared to SnO2/ LTOAr CHFs. Additionally, the presence of O-C]O was detected in SnO2/LTOAr hollow fibers at 288.91 eV [43]. In summary, SnO2/LTOA CHFs were confirmed with the presence of both SnO2 and LTO phases, whereas a lower binding energy in Ti-O bond of SnO2/LTOAr CHFs confirms a valance reduction of Ti to þ3. The presence of Ti3þ in SnO2/LTOAr CHFs was advantageous as it can contribute to high electronic conductivity and faster Li ion intercalation. Thus, the presence of SnO2 and LTO in both the CHFs was confirmed.

3.3. Surface area and porosity of SnO2/LTO CHFs Surface area measurements were carried out for understanding the porosity, pore size and pore volume in the CHFs and the results were summarized in Table 1. Nitrogen adsorption-desorption isotherms of SnO2/LTOA and SnO2/LTOAr CHFs in Fig. 4A represent type IV isotherms with type III [45] and type H2 [10,14,46] hysteresis loops respectively, that evidenced the mesoporous nature. In SnO2/LTOAr CHFs closure of isotherm loop around 0.4 relative pressures (P/P0) confirmed the presence of pore less than 10 nm in size [47]. SnO2/LTOAr CHFs showed a higher hysteresis than SnO2/ LTOA CHFs pertaining to a more porous nature than the later (Fig. 4A). The BET surface areas for SnO2/LTOA and SnO2/LTOAr CHFs were measured as 12.55 m2/g and 102.40 m2/g respectively. On analyzing the pore size distribution by BJH method, SnO2/LTOA CHFs were perceived with more uniform sized pores than in SnO2/ LTOAr CHFs (Fig. 4B). The BJH pore size and pore volumes of SnO2/ LTOA and SnO2/LTOAr CHFs were measured as 28.8, 4.64 nm and 0.09, 0.11 cm3/g respectively. The BET surface area and BJH pore volume results were in accordance with the TEM images (Fig. 2B1 and B2) that confirmed the presence of small sized grains in SnO2/LTOAr CHFs providing higher surface area and porosity when compared with SnO2/LTOA CHFs. In the nutshell high surface area and porosity observed in both CHFs can aid in reducing the lithium ion diffusion length and also ease in accommodating the huge volume expansion during Li insertion. To validate this, electrochemical studies were performed and were discussed in following sessions.

Scheme 2.


Scheme 3.

Scheme 4.

Scheme 5.

3.4. Cyclic voltammetry studies Cyclic voltammetry studies were conducted for the CHFs in the voltage range of 0.05e2 V at a scan rate of 0.02 mV/s (Fig. 5). During the initial discharge cycle which started from the cell open circuit voltage (OCV) 2.5 V, both types of CHFs have shown reduction peaks at 1.54, 1.46, 1.02, 0.5, 0.3 and 0.2 V respectively. First and last reduction (cathodic) peaks in Fig. 5 was due to the respective reduction of Ti4þ to Ti3þ and formation of SnLi alloy during Li intercalation [27,48,49]. The remaining irreversible cathodic peaks are due to the Solid Electrolyte Interphase (SEI) layer formed by the decomposition of electrolyte to Li2O and conversion of SnO2 to Sn. The oxidation peaks at 0.5 V represented the Sn (de-alloying) during cell discharge. The oxidation peaks at 1.3 and 1.59 V were accredited to the formation of SnO2 and oxidation of Ti3þ to Ti4þ respectively. The second and third cycles showed reduction peaks at 1.54 V, 0.2 V and oxidation peaks at 1.59 V, 0.5 V which were for Ti redox couple and Sn alloying/de-alloying couples respectively, attributing the reversible nature of both the CHF electrodes. The mechanism of Li cycling with both CHF electrodes can be explained as combination mechanisms of conversion and intercalation in the first cell discharge as shown in the following Schemes 2e4. Later the alloy/de-alloy mechanism of Sn dominates and contributed to the reversible capacity of the electrodes (in the voltage range 0.05e2 V) as follows [9,50].(see Scheme 5) 3.5. Galvanostatic charge edischarge studies In order to compare the charge/discharge stability performance, SnO2 bare fibers, SnO2/C, SnO2/LTOA CHFs and SnO2/LTOAr CHFs were cycled at 0.1 C for 11 cycles from 0.05 to 1.5 V respectively (Fig. S6). The charge-discharge profiles of the four fiber samples have shown high capacities in the first cycle. For SnO2 fibers a clear plateau was observed at 0.8e1.2 V indicating the reduction of Sn4þ to Sn0 and formation of Li2O [51]. The reduction reactions of SnO2 are given in Schemes 3 and 4. The same phenomenon was observed with SnO2/C, SnO2/LTOA and SnO2/LTOAr CHFs respectively (Fig. S6B, C & D). SnO2 bare fibers have experienced drastic capacity fading in the first 10 charge-discharge cycles at 0.1 C rate (Fig. S6A). This was due to their large volume expansion and structure disintegration experienced while alloying and de-alloying with lithium. For SnO2/ C fibers a slight improvement in cycle stability was perceived whereas, SnO2/LTOA and SnO2/LTOAr CHFs showed higher cyclic stability with less capacity fading (Fig. S6B and D). For a better comparative study, the coulombic efficiency of the four fiber samples was provided in Fig. S7. The bare SnO2 fibers and SnO2/C fibers were detected with an initial columbic efficiency of 27.9% and 28.4% respectively. Meanwhile, the first cycle columbic


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LTOA fibers were observed with a capacity loss of 7% from the initial value. Further, when the charge-discharge studies was continued at 1 C rate for 110 cycles, SnO2/LTOA CHFs were perceived with significant capacity fading from 300 to 150 mAh/g (Fig. 6B) providing only 50% capacity retention. Whereas, SnO2/LTOAr CHFs were observed with a very little capacity fading (from 330 to 310 mAh/g) having almost 90% capacity retention (Fig. 6D). Moreover a comparative study of the battery performances of SnO2 LTO based electrodes, with the available literature is provided in Table 3. SnO2/LTO CHFs prepared in our study was observed as SnO2 fibers > SnO2 =C fibers > SnO2 =LTOA CHFs > SnO2 =LTOAr CHFs superior to the reported literature in terms of specific capacity values, Cycle stability as well as percentage capacity retention. Also, among the two CHFs synthesised in our study, the reason for suThe improved cyclic performance of both CHFs can be ascribed perior rate capability and cycle stability for SnO2/LTOAr CHFs can be to their porous and hollow architecture with smaller grain sizes and as due to their smaller particle size, higher surface area and larger better structural integrity due to the presence of electrochemically pore volume that enabled a lower lithium ion diffusion length than stable LTO grains (zero crystal stain) in the neighbourhood of SnO2 the latter. Moreover, the presence of Ti3þ in SnO2/LTOAr CHFs might particles. have provided higher electronic and ionic conduction enabling easy For further understanding the high rate capability of the SnO2/ and fast alloying of Li ions when compared with the latter LTOA and SnO2/LTOAr CHF electrodes, charge-discharge studies [14,15,17]. For validating the above sentence electrochemical were conducted at higher C rates in the order of 0.3, 0.5, 0.8, 1, 2 and impedance studies were conducted for SnO2/LTOA and SnO2/LTOAr 5 C (Fig. 6). When compared with SnO2/LTOA CHF, SnO2/LTOAr CHF CHFs. were perceived with greater cycle stability and lower capacity

efficiency SnO2/LTOA CHFs and SnO2/LTOAr CHFs were observed as 36% and 46% respectively. As the charge-discharge proceeded to 11 cycles, the coulombic efficiency values in all fiber samples were observed as increasing. The 11th cycle columbic efficiency for the SnO2 and SnO2/C fibers were perceived as 83% and 91% respectively. At same time, at the end of 11th cycle 97, 93% columbic efficiency was observed for SnO2/LTOAr and SnO2/LTOA CHFs respectively. The electrochemical performance in the four electrodes can be represented in the following order:

fading as the charging rates of the cells were increased (Fig. 6A and C). For instance at charging rates of 0.3, 0.5, 0.8, 1, 2 and 5 C, SnO2/ LTOA CHFs have retained discharge capacities of 540, 452, 417, 362, 320, 250 and 145 mAh/g respectively whereas, SnO2/LTOAr CHFs were observed with 435, 390, 363, 350, 330, 320 and 270 mAh/g. For a better comparative study, charge-discharge capacities of both CHFs at various C rates have been tabulated in Table 2. At charging rates higher than 1 C SnO2/LTOACHFs were observed with a slight capacity fading in 10 cycles whereas, SnO2/LTOAr CHF delivered 100% capacity retention. After the sequential rate performance study up to 5 C and when charged back to 1 C, SnO2/LTOAr CHFs could retain the same capacity (330 mAh/g), however SnO2/

3.6. Electrochemical impedance studies Nyquist plot were recoded for SnO2/LTOA and SnO2/LTOAr CHFs after 10 (Fig. 7A) and 110 (Fig. 7B) charge-discharge cycles. The presence of electrolyte resistance (Re), surface layer resistance (Rsl) and charge transfer resistance (Rct) were detected in the Nyquist plots representing two semicircles indicating two time constants. Electrochemical impedance circuit fit results of both types of composite fibers were summarized in Table 4. The semicircle that appeared in the higher frequency region was due to the resistance of surface layer formed on the electrode and the one in the lower

Fig. 6. Galvanostatic charge discharge studies of SnO2/LTOA and SnO2/LTOAr CHF at varying C rates (A and C). 110 cycles of charge discharge stability studies of SnO2/LTOA (B) and SnO2LTOAr CHFs (D) at 1 C rate.

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Table 2 Discharge capacities of SnO2/LTO CHFs (after first 10 cycles) at various C rates. Composite hollow fiber samples

1st and 10th cycle specific capacity (mAh/g) of samples at various C rates 0.1 C


0.5 C

















1400 1475

540 435

417 363

410 360

320 331

318 330

250 320

248 320

145 270

144 270

300 330

296 330

Table 3 A competitive study on the battery performance of CHFs with SnO2-LTO electrodes from literature. [Ref]

Type of composite electrode, synthesis method

ICa (mAh/g)

Long term cycling studies Capacity (mAh/g)

Charge density/C rate

Cycle number

[23] [24] [25] [26] [27] [28] [29] Our work

Spray drying (hollow spheres) LTO coated hollow SnO2 particles Combustion SnO2 coated LTO powders Sol-gel (powders) Solution precipitation (powders) Spray pyrolysis (powders) Sol-gel/electrospinning (Hollow fibers)

530e700 e 300e442 476 688 600e700 380e550 430e540

350 439 100e150 276 642 189 271e312 150e310

170 mA g1 1000 mAg1 100 mAg1 0.5 mA/cm1 0.2 C 0.5 mAcm1 700 mAg1 1 Ce780 mAg1

30 30 50 16 60 42 60 110

a b

CRb (%)

70e82 e 60e95 e 94.3 78e88 90

Initial Capacity. Capacity retention.

Fig. 7. Nyquist plots showing electrochemical impedance studies of SnO2/LTOA and SnO2/LTOAr CHF electrodes after 10 cycles (A) and 110 cycles (B) of charge-discharge studies.

Table 4 Summary of Electrochemical impedance results. Sl. No

Hollow fiber samples

Re (U)

Rsl (U)

Rct (U)

1 2 3 4

SnO2/LTOACHFs (10 cycles) SnO2/LTOA CHFs (110 cycles) SnO2/LTOAr CHFs (10 cycles) SnO2/LTOAr CHFs (110 cycles)

8.09 14.73 9.31 11.29

5593 121 10154 113.8

8.09 987 44654 309.3

frequency region was ascribed to the charge transfer resistance of the electrode material. An increase in the Re electrolyte resistance value was observed after 110 cycles of charge discharge (when compared with the 10 cycle Re) in both the CHFs demonstrating the building up of SEI layer. SnO2/LTOAr CHFs was spectated with formation of low SEI layer according to the low Re values. Moreover, Rct in both the CHFs have witnessed lower value after 110 cycles representing fast Li alloying/de-alloying. This was described as follows: During long cycling, SEI provided the conductive paths for lithium ions by reducing the overall cell resistance. Among the CHFs, SnO2/LTOAr evidenced superior performance with lower Re (8.29 U) and Rsl (113 U) in comparison to SnO2/LTOA after 110 cycles. Further SnO2/ LTOAr CHFs was detected with Rct values almost three times lower (306 U) than SnO2/LTOA CHFs (987 U). The lower Rct observed with

SnO2/LTOAr CHFs was accredited to the presence of conductive Ti3þ, smaller grain sizes as well as high surface areas than SnO2/LTOA CHFs, which were already confirmed by the XPS, TEM images and BET surface area measurements. Similarly the lower Rsl in the SnO2/ LTOAr CHFs was ascribed to the lower growth of surface layer after each charge-discharge cycle.

3.7. Complete Li-ion cell (full cell) using SnO2/LTOA CHF anode and LFP cathode For understanding the working of CHF in a full Li ion cell, the better performed SnO2/LTOAr CHF anode in half cell was chosen for full cell studies. SnO2/LTOAr CHF anode was assembled with commercially available LiFePO4 (LFP) cathode. The anode to cathode weight ratio was maintained 1:3 in order to compensate for the Li loss in SEI. Fig. 8A illustrates the GCD profile for LFP/Li, SnO2/LTOA CHFs/Li and SnO2/LTOA CHFs -LFP cells charged-discharged at 0.1 C rates. LFP half-cell was observed with a voltage of 3.5 V and specific capacity 161 mAh/g whereas SnO2/LTOA CHF half-cell was perceived with a voltage of 0.5 V and 600 mAh/g specific capacity. The SnO2/LTOA CHFs eLFP full cell has delivered a voltage of 3 V and specific capacity 230 mAh/g. In comparison with the individual


A.K. Haridas et al. / Materials Today Energy 4 (2017) 14e24

Fig. 8. (A) Galvanostatic charge discharge profiles showing voltage and specific capacities of LFP- Li, SnO2/LTOAr CHF -Li and SnO2/LTOAr CHF eLFP cells at 0.1 C rates. (B) Galvanostatic charge discharge profiles of SnO2/LTOAr CHF eLFP cells in 1st and 20th cycle at 0.1 C rates.

Fig. 9. Galvanostatic charge discharge stability performance and coulombic efficiency plot of SnO2/LTOAr CHF-LFP full cell at 0.1 C rate in 20 cycles.

half-cells, the SnO2/LTOA CHFs eLFP full cell possesses high energy density as the voltage and capacity delivered are in the higher range. The full cell was charged-discharged for 20 cycles to understand the capacity stability. GCD profile of SnO2/LTOA CHFs eLFP full cell at 1st and 20th cycles was shown Fig. 8B. Even though the first cycle of full cell delivered a charge capacity of 550 mAh/g discharge capacity were about 200 mAh/g correlating to a low coulombic efficiency. On proceeding further cycling, the coulombic efficiency of the full cell was observed as increasing. The 20th cycle discharge capacity was observed 230 mAh/g. The cycle stability and coulombic efficiency of SnO2/LTOA CHFs eLFP full cell for 20 cycles are given in Fig. 9. An increase in the discharge capacity was observed with the full cell from 1st to 20th cycle. The percentage increase in discharge capacity in 20 cycles can be calculated as 15%. Moreover, an increase in coulombic efficiency of full cell was observed (92%) at the end of 20th cycle. In graphite eLFP full cell that can deliver 3 V and ideally a capacity of 372 mAh/g w. r.t the mass of anode, an energy density of 1116 Wh/kg can be achieved. By considering the reduction factor of

1/3 for the weight of current collector, electrolyte and aluminum case [52] a practical energy density of 372 Wh/kg can be obtained. Further by considering an additional specific capacity due to weight reduction in high capacity SnO2/LTOAr CHF anode which can offer ~600 mAh/g at 0.1 C in half cell (graphite 372 mAh/g), the SnO2/ LTOAr CHF eLFP full cell can account for an energy density higher than 372 Wh/kg. Hence SnO2/LTO CHFs can be considered as a contender for high capacity anodes similar to Sn and Si for use in high power LIBs.

4. Conclusions Sol-gel/electrospinning route was selected for fabricating SnO2/ LTOA and SnO2/LTOAr CHF due to its cost effectiveness, ease and reproducibility. The presence of LTO in these CHF has aided in controlling the grain growth of SnO2 during heat treatment, creating nano -sized grains. The FESEM and TEM images of CHFs confirmed their hollow nature with grain sizes 30 nm and 5e20 nm for SnO2/LTOA and SnO2/LTOAr CHFs respectively. XPS studies

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revealed the presence of SnO2 and LTO in both the CHFs with a slight reduction in oxidation state for Ti (Ti3þ) in SnO2/LTOAr CHFs indicating more electronic conduction. Both the CHFs were observed with high rate capability and cycle stability in comparison with the electrospun bare SnO2 and SnO2/C fibers. Moreover, the porous and hollow nature of fibers has also abetted in controlling the volume expansion and pulverization in SnO2 during alloyingde-alloying reactions, and exhibited high rate performances. Further, due to the presence of conductive Ti3þ, SnO2/LTOAr CHFs provided excellent rate capability in comparison with SnO2/LTOA CHFs with 90% capacity retention after 110 cycles of chargedischarge. Moreover, low resistances were observed for the same when compare with SnO2/LTOA CHFs in the electrochemical impedance measurements. On attempting the full cell study of SnO2/LTOAr CHFs with commercial LFP a cell voltage of 3 V and specific capacity of 230 mAh/g were obtained at 0.1 C rates. Like the other high capacity anodes Si, SiO2, Sn and SnO2; SnO2/LTOAr CHF can be used for high energy application in combination with high capacity cathodes. Acknowledgments TNR and AKH acknowledge DST Nanomission, India for financial support. Authors acknowledge Dr Shanti V. Nair and Dr Sajini Sreekumar of Amrita centre for Nanosciences, Cochin, India in TEM imaging. We also extent our sincere thanks to Mrs A. Jyothirmayi, ARCI, Hyderabad for Impedance measurements. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.mtener.2017.01.002. References [1] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (2013) 5364e5457. [2] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Li-ion battery Materials: present and future, Mat. Today 18 (2014) 252e264. [3] J.S. Chen, X. Wen, D. Lou, SnO2 -based Nanomaterials: synthesis and application in lithium-ion batteries, Small 11 (2013) 1877e1893. [4] J. Zhang, L.B. Chen, C.C. Li, T.H. Wang, Amorphous SnO2 e SiO2 thin films with reticular porous morphology for lithium-ion batteries, Appl. Phys. Lett. 93 (2008) 1e4. [5] J. Zhu, Z. Lu, S.T. Aruna, D. Aurbach, A. Gedanken, Sonochemical synthesis of SnO2 nanoparticles and their preliminary study as Li insertion electrodes, Chemi. Mater 12 (2000) 2557e2566. [6] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Highperformance lithium battery anodes using silicon nanowires, Nat. Nanotechnol. 3 (2008) 31e35. [7] H. Wu, G. Chan, J.W. Choi, I. Ryu, Y. Yao, M.T. Mcdowell, S. Woo, A. Jackson, L. Hu, Y. Cui, Stable cycling of double-walled silicon nanotube battery anodes through solideelectrolyte Interphase control, Nat. Nanotechnol. 7 (2012) 310e315. [8] N.-S. Choi, Y. Yao, Y. Cui, J. Cho, One dimensional Si/Sn - based nanowires and nanotubes for lithium-ion energy storage materials, J. Mat. Chem. 21 (2011) 9825e9840. [9] P. Meduri, C. Pendyala, V. Kumar, G.U. Sumanasekera, M.K. Sunkara, Hybrid tin oxide nanowires as stable and high capacity anodes for Li-Ion batteries, Nano Lett. 9 (2009) 612e616. [10] H. Wang, G. Liu, Z. Yang, B. Wang, L. Chen, Q. Jiang, Facile synthesis of mesoporous SnO2 submicrospheres by microemulsion approach as high capacity anodes material for lithium-ion batteries, Int. J. Electrochem. Sci. 8 (2013) 2345e2353. [11] S. Chen, M. Wang, J. Ye, J. Cai, Y. Ma, H. Zhou, L. Qi, Kinetics-controlled growth of aligned mesocrystalline SnO2 nanorod arrays for lithium-ion batteries with superior rate performance, Nano Res. 6 (2013) 243e252. [12] R. Liu, N. Li, D. Li, G. Xia, Y. Zhu, S. Yu, C. Wang, Template-free synthesis of SnO2 hollow microspheres as anode material for lithium-ion battery, Mat. Lett. 73 (2012) 1e3. [13] L. Zou, L. Gan, F. Kang, M. Wang, W. Shen, Z. Huang, Sn/C non-woven film prepared by electrospinning as anode materials for lithium ion batteries, J. Power sources 195 (2010) 1216e1220.


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