Sn al2o3 c cnt composite prepared by wet milling as anode material for lithium ion cells

Journal of Science: Advanced Materials and Devices (2017) 210e214 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Sn/Al2O3/C/CNT composite prepared by wet milling as anode material for lithium-ion cells C.P Sandhya a, Bibin John a, *, C Gouri b a b Energy Systems Division, Vikram Sarabhai Space Centre, Thiruvananthapuram-695022, Kerala, India Polymers and Special Chemicals Group, Vikram Sarabhai Space Centre, Thiruvananthapuram-695022, Kerala, India a r t i c l e i n f o a b s t r a c t Article history: Received 20 February 2017 Received in revised form 11 April 2017 Accepted 14 April 2017 Available online 23 April 2017 Sn/Al2O3/C/CNT (SAC/CNT) composite was synthesized by a simple wet milling route The physicochemical, structural and morphological properties of the material were studied The electrochemical performance of the composite as an anode material in Li-ion cells was evaluated by Cyclic Voltammetry (CV), chargeedischarge cycling and electrochemical impedance measurements The SAC/CNT material delivered an initial specific capacity of 835 mAh gÀ1 with the coulombic efficiency of ~77% along with good chargeedischarge cycle performance retaining ~88% of the initial capacity after 35 cycles A comparison of the results with those for Sn/C (SC) and Sn/Al2O3/C (SAC) showed that the SAC/CNT composite exhibited better overall performance compared to the other two materials The enhanced performance of SAC/CNT is attributed to the combined effect of the buffer action provided by Al2O3 accommodating volume changes of the electrode during cycling and the reduced charge transfer resistance of the electrode resulting from the inclusion of conductive CNTs © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Tin Anode Carbon nanotubes Lithium-ion cells Introduction Li-ion cells with high specific energy and good cycle performance are gaining immense importance as rechargeable energy sources for various applications Graphite based anode materials are commonly employed in Li-ion cells Being limited in capacity (372 mAh gÀ1), graphite needs to be replaced with high capacity alternatives to attain high energy systems [1] Sn with a high theoretical capacity of 993 mAh gÀ1 holds promise as an alternate anode material based on its alloying reaction with Li [2] However, as a result of large volume changes during the alloying and dealloying process of Sn with Li, electrical disconnection of active material from the current collector occurs during repeated chargeedischarge cycling and leads to poor cyclability [3] Several strategies are being studied in detail to improve the mechanical stability of Sn-based electrodes which include the use of nano-sized active material [4], inter-metallic compounds [5], Sn/ C nanotubes [6], and Sn-inactive matrix composites [7] The incorporation of an electrochemically inactive material like Al2O3 as a matrix in the electrode, is reported to be effective as a buffer * Corresponding author E-mail address: bbnjohn@yahoo.com (B John) Peer review under responsibility of Vietnam National University, Hanoi that can accommodate the huge volume changes during cycling Lee et al [7] reported the preparation of Sn2Fe/Al2O3/C nanocomposite by high energy, mechanical milling method that retained a capacity of 477 mAh gÀ1 after 40 cycles Here, the embedded Al2O3 improved the mechanical stability of the material and consequently enhanced the cycle performance Another way to improve the performance of Sn is to incorporate the carbon layer over Sn or the active material in to a carbon matrix [8,9] Generally, the carbon over-layer and carbon matrix not only offer better electrical conductivities, but also buffer the volumetric change of Sn during cycling; thus improving the cycle performance of Sn anode [10,11] Among the various forms of carbon, single walled carbon nanotubes (SWCNTs) attract attentions due to their onedimensional structure with high length-to-diameter ratio, high electrical conductivity combined with high porosity and surface area [12e14] Noerochim et al reported the study on SnO2/SWCNT electrode wherein the nanosized SnO2 provided high capacity and SWCNT acted as a flexible mechanical support for strain release, offering an efficient electrically conducting channel [15] The material showed good retention of properties during cycling with a specific capacity of 454 mAh gÀ1, tested after 100 cycles at a current density of 25 mA gÀ1 In this work, a simple wet milling route was adopted for the synthesis of active material Sn based electrodes Sn/C (SC), Sn/ http://dx.doi.org/10.1016/j.jsamd.2017.04.003 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) C.P Sandhya et al / Journal of Science: Advanced Materials and Devices (2017) 210e214 Al2O3/C (SAC) and Sn/Al2O3/C/CNT (SAC/CNT) composites were prepared, characterized and electrochemical characteristics of these materials as anode in Li-ion cells were evaluated and compared The effectiveness of Al2O3 and SWCNT addition in the electrode composition towards arresting the volume change and improving the charge transfer characteristics of Sn based anode is demonstrated in this study Experimental 2.1 Materials Sn powder (99%, Aldrich, 10 mm), Al2O3 (Otto reagents), SWCNT (diameter: ~1.5 nm, length: 1e5 mm, purity>95%, from Nanolab), and acetylene black (Timcal, Switzerland) were used for the synthesis of composites Toluene (Nice Analytical reagents) was used as the medium for wet milling Other chemicals and materials used were: poly vinylidene fluoride (PVdF) and N-methylpyrrolidone (NMP) (both from Sigma Aldrich), copper foil (10 mm thickness, Schlenk Metallfolien, Germany), electrolyte M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC):ethyl methyl carbonate (EMC) (1:1:1, by weight, Danvec, Singapore), celgard 2320 separator (PP/PE/PP trilayer membrane, 20 mm thickness, Celgard, USA), and lithium metal foil (Aldrich) 2.2 Preparation of SC, SAC and SAC/CNT composites Sn powder and acetylene black were taken in the ratio of 9:1 by weight (total about g) in a zirconia jar and ball milled (wet milling in presence of toluene) at 500 rpm for 12 h After milling, the mixture was allowed to settle down and the solvent was decanted off The resultant mixture was vacuum dried at 100  C for 12 h to obtain the active material noted as SC The same procedure was adopted for the synthesis of SAC composite in which Sn, Al2O3 and acetylene black were taken in the ratio of 8:1:1, and SAC/CNT in which Sn powder, Al2O3, acetylene black and SWCNT were taken in the ratio of 8:1:0.5:0.5 The total quantity taken was approximately g for all the three samples 2.3 Characterization of composites the voltage range of 0.01e2.50 V vs Li/Liỵ at room temperature using an eight channel, Bitrode button cell cycling system (Model: MCV8-1/0.01/0.001-5B) Impedance measurements were done on Solartron e SI 1260 Impedance/Gain e Phase analyser Results and discussion 3.1 Synthesis of the Sn/Al2O3/C composites In the present study, the focus was to investigate the effect of Al2O3 and CNT on the performance of Sn based electrodes for Li-ion cells For this purpose, three active composites with Sn, i.e SC, SAC and SAC/CNT were synthesized, as detailed in subsection 2.2 The characterization of the composites and their electrochemical evaluation are discussed in subsequent sections 3.2 Characterization of composites The XRD patterns of the synthesized composites SC, SAC and SAC/CNT are presented in Fig 1(a)e(c), respectively All the peaks correspond to pure Sn phase in all the three samples (JCPDS 00004-0673) The sharp diffraction peaks at 30 , 32 , 44 , 45 , 55 , 62 , 63 , 65 , 72 , 73 , 79 and 89 are ascribed to the (200), (101), (220), (211), (301), (112), (400), (321), (420), (411), (312) and (431) diffraction peaks of Sn [16,17] The absence of peaks corresponding to any other alloy phase and non-shifting of the Sn peaks in all the three composites confirm that the added components Al2O3, C and SWCNT act as separate constituents and not cause any structural change to Sn phase in the composite The average crystallite size of the samples is calculated with respect to the diffraction peak at 32 using the Scherrer equation: D ¼ 0.9l/bcosq; where D is the average crystallite size, l is the wavelength of CuKa, b is the full width at half maximum of the diffraction peaks, and q is the Bragg's angle The average crystallite sizes calculated for Sn in the three composite materials are similar, in the range of 800 nm The SEM images of the composite samples are shown in Fig It is observed that the particles formed vary widely in sizes, ranging from sub-micron to a few microns, but mostly in mme2 mm range In addition, the tendency to exist as particle agglomerates is noted The phase composition of the synthesized materials was examined by X-ray diffraction (XRD) (Panalytical X0 Pert PRO, Philips diffractometer) with a CuKa radiation source The microscopic features of the samples were examined using scanning electron microscopy (SEM) (INCA Penta FETX3, EVO 50) and transmission electron microscopy (TEM) (Technai 30 G2, S-Twin) The selected area electron diffraction pattern (SAED) was also derived from TEM analyses 2.4 Electrochemical evaluation The electrode was prepared by blending a mixture of the active material (SC, SAC or SAC/CNT) with PVdF binder in the ratio of 85:15 using NMP to form slurry The slurry was then coated over copper foil, dried at 100  C in air for h and under vacuum for 10 h The electrodes thus obtained (having a diameter of 12 mm) were evaluated for electrochemical performance vs lithium metal by assembling coin cells (CR 2032) with celgard 2320 separator and M LiPF6 in EC:DEC:EMC electrolyte The cells were assembled in a glove box filled with dry argon gas (H2O < ppm, O2 < ppm, Mbraun, Germany) For electrochemical performance evaluation of active materials, constant current dischargeecharge and impedance measurements were carried out for the fabricated coin cells The chargeedischarge cycling of the cells was performed within 211 Fig XRD patterns of SC (a), SAC (b), and SAC/CNT (b) composites 212 C.P Sandhya et al / Journal of Science: Advanced Materials and Devices (2017) 210e214 Fig SEM images of SC (a), SAC (b), and SAC/CNT (c) composites The synthesis route adopted, i.e ball milling of the components, causes intimate mixing of the ingredients and lead to particle coalescence The varying degree of coalescence resulted in different particle sizes and shapes The SAC/CNT composite material comprising of SWCNT was subjected to bright field TEM analysis and the images obtained at different magnifications are shown in Fig The dark spots indicate metallic Sn particles that are distributed in the carbon and Al2O3 matrix The presence of SWCNT in the composite is observed clearly in the higher magnification TEM image, as shown in Fig 3(b) The crystalline nature of Sn in the composite material is evident in Fig 3(c) and the corresponding SAED pattern is shown in the inset of Fig 3(c) with a calculated d spacing of 1.5 Å 3.3 Electrochemical performance evaluation The electrochemical performances of the composite electrodes were studied by chargeedischarge cycling of the electrodes at C/15 rate in the voltage range 0.01e2.50 V vs Liỵ/Li Fig 4(a) shows the first cycle dischargeecharge curves for SC (mass of active material: Fig TEM images of SAC/CNT at different magnifications Inset of Fig (c) is the SAED pattern corresponding to the image (c) C.P Sandhya et al / Journal of Science: Advanced Materials and Devices (2017) 210e214 213 Table Data pertaining to the cycle performance of SC, SAC and SAC/CNT electrodes Electrode Cycle No Specific capacity, mAh gÀ1 SC SAC SAC/CNT Fig (a) Typical dischargeecharge voltage profiles for SC, SAC and SAC/CNT composite electrodes; (b) Cycle performance of SC, SAC and SAC/CNT composite electrodes 0.90 mg cmÀ2), SAC (mass of active material: 0.93 mg cmÀ2) and SAC/CNT (mass of active material: 0.91 mg cmÀ2) composites The SAC/CNT composite delivers 1148 mAh gÀ1 and 838 mAh gÀ1 capacities for the first discharge and charge respectively, with a coulombic efficiency of 73% The excess capacity observed in the discharge process (theoretical capacity of Sn is 993 mAh gÀ1) appears to originate from electrolyte decomposition, and thus, perhaps the subsequent formation of an organic layer on the surface of the particles [18] The efficiency got improved in the subsequent cycles due to stabilization of the solid electrolyte interface formed In comparison, it is seen that the first cycle charge capacity for SAC/CNT composite is higher than that for SAC and SC composites, showing 729 and 567 mAh gÀ1, respectively as shown in Fig 4(a) Fig 4(b) shows the cycle performance of composite electrodes for 35 chargeedischarge cycles, at a rate of C/15 between 0.01 and 2.50 V The data generated from the chargeedischarge curves on charge capacity, discharge capacity, coulombic efficiency and retention of charge capacity at the 20th and 35th cycles with respect to the first cycle capacity are consolidated in Table The performances of all the materials decrease with increase in cycle number However, the decrease is rapid in the case of SC; thus, there is a retention of only about 36% charge capacity after 35 cycles Here, the large volume change of the active material is causing a detrimental effect on cycling Compared to the performance of SC, SAC shows a lesser fade in the property with cycling Thus, after 35 cycles, 72% of its initial capacity is retained This improvement may be due to the presence of buffer matrix Al2O3, which can arrest and accommodate the drastic volume changes effectively In the case of SAC/CNT composite, the decrease in capacity with cycle number is gradual and much less compared to the other two materials After 20 35 20 35 20 35 Discharge Charge 824 386 225 1000 634 566 1148 823 789 567 351 205 729 590 527 838 765 734 Retention of Coulombic initial charge efficiency, % capacity, % e 62 36 e 81 72 e 91 88 69 91 91 73 93 93 73 93 93 35 cycles, SAC/CNT still retains a charge capacity of 734 mAh gÀ1 with 88% capacity retention This property enhancement can be attributed to two factors: (i) the buffer action from Al2O3 against volume expansion of Sn, and (ii) the enhanced conductivity imparted by SWCNT to the active material reducing the charge transfer resistance, as evidenced from impedance measurements (discussed further) For a better understanding of the effect of SWCNT addition on the conductivity, electrochemical impedance studies were carried out on all the three composite electrodes; the results are presented in Fig It can be observed that the impedance curves consist of one compressed semicircle in the medium-frequency region and an inclined line in the low-frequency region [10,19] The impedance plots were fitted using the equivalent circuit model, as shown in the inset of Fig 5, which includes the ohmic resistance RU (total resistance of the electrolyte, separator, and electrical contacts), the charge-transfer resistance Rct, the Warburg impedance (W) related to the diffusion of Liỵ ions into the bulk of the electrode materials, and constant phase element (CPE1) associated with the interfacial capacitance As illustrated, the diameter of the semicircle for the SAC/CNT composite is much smaller than that of SAC and SC, revealing lower charge transfer resistance The Rct value was 14 U for the SAC/CNT electrode, which is significantly lower than 48 U for the SAC electrode and 60 U for SC electrode, indirectly implying the enhanced electronic conductivity of SAC/CNT electrode achieved through the efficient electrically conducting channel provided by SWCNT In addition, as reported in earlier studies, SWCNT can function as a flexible mechanical support for the electrode and help to release strain [15] Fig Electrochemical impedance results of SC, SAC and SAC/CNT composite electrodes Equivalent circuit model is given as inset 214 C.P Sandhya et al / Journal of Science: Advanced Materials and Devices (2017) 210e214 The results from this study demonstrate that the main drawback of large volume change during cycling of Sn-based active materials leading to poor cyclability can be effectively addressed by the incorporation of two ingredients in the electrode composite: a buffer matrix like Al2O3, and electronic conductor like SWCNT Also, the study shows that the easy, simple method of wet ball milling can be employed to prepare the composite electrode which performs as a promising anode material for Li-ion cells [3] [4] [5] [6] Conclusion Sn/Al2O3/C/CNT (SAC/CNT) composite was synthesized by the wet milling method and evaluated for performance as an anode material for Li-ion cells The SAC/CNT composite delivered a charge capacity of 838 mAh gÀ1 at C/15 rate and retained 88% of its initial capacity after 35 dischargeecharge cycles Sn/carbon (SC) and Sn/ Al2O3/carbon (SAC) composites were also prepared under identical conditions and evaluated The order of performance in terms of specific capacity as well as cycle performance during 35 dischargeecharge cycles observed was SAC/CNT > SAC > SC The higher performance of SAC vs SC is attributed to the buffer effect arising from the inactive matrix Al2O3 against large volume change of Sn In the case of SAC/CNT, in addition to the effect of Al2O3, the lowering of charge-transfer resistance arising from the inclusion of electronic conductor SWCNT leads to further improvement in performance Acknowledgements [7] [8] [9] [10] [11] [12] [13] [14] The authors thank Director, VSSC for the permission granted to publish this article Material Characterization Division, VSSC is acknowledged for their support in various analyses Sandhya C.P thanks Council of Scientific and Industrial Research (CSIR) for the financial support for doing the research [15] References [18] [1] T Tatsuma, M Taguchi, N Oyama, Inhibition effect of covalently cross-linked gel electrolytes on lithium dendrite formation, Electrochim Acta 46 (2001) 1201e1205 [2] A Sivashanmugam, T Premkumar, S Gopukumar, N.G Renganathan, M Wohlfahrt-Mehrens, J Garche, Synthesis and electrochemical behavior of [16] [17] [19] tin oxide for use as anode in lithium rechargeable batteries, J Appl Electrochem 35 (2005) 1045e1050 X.W Lou, Y Wang, C.L Yuan, J.Y Lee, L.A Archer, Template-free synthesis of SnO2 hollow nanostructures with high lithium storage capacity, Adv Mater 18 (2006) 2325e2329 Y Liang, J Fan, X Xia, Z Jia, Synthesis and characterization 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SC, SAC and SAC /CNT electrodes Electrode Cycle No Speci? ?c capacity, mAh gÀ1 SC SAC SAC /CNT Fig (a) Typical dischargeecharge voltage profiles for SC, SAC and SAC /CNT composite electrodes; (b) Cycle... promising anode material for Li -ion cells [3] [4] [5] [6] Conclusion Sn/ Al2O3/ C/ CNT (SAC /CNT) composite was synthesized by the wet milling method and evaluated for performance as an anode material for. .. Cycle performance of SC, SAC and SAC /CNT composite electrodes 0.90 mg cmÀ2), SAC (mass of active material: 0.93 mg cmÀ2) and SAC /CNT (mass of active material: 0.91 mg cmÀ2) composites The SAC/CNT