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High performance titanium antimonide TiSb2 alloy for Na-ion Batteries and Capacitors María Arnaiz, Juan Luis Gómez-Cámer, Jon Ajuria, Francisco Bonilla, Begoña Acebedo, María Jáuregui, Eider Goikolea, Montserrat Galceran, and Teófilo Rojo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02639 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018
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Chemistry of Materials
High performance titanium antimonide TiSb2 alloy for Na-ion Batteries and Capacitors María Arnaiz†,§, Juan Luis Gómez-Cámer†,§,*, Jon Ajuria†, Francisco Bonilla†, Begoña Acebedo†, María Jáuregui†, Eider Goikolea‡, Montserrat Galceran†, Teófilo Rojo†, ‡, * † CIC
energiGUNE, Albert Einstein 48, Technology Park of Álava, 01510 Miñano (Álava), Basque Country, Spain
‡ Inorganic
Chemistry Department, University of the Basque Country UPV/EHU, P.O. Box 644, 48080 Leioa, Spain
ABSTRACT: Herein, we report for the first time the use of TiSb2 alloy as anode material for sodium-ion batteries (NIBs) and capacitors (NICs). The electrochemical performance of TiSb2 in NIBs shows stable cycling with a capacity of about 225 mAh·g-1 for 200 cycles. Discrepancies with the expected theoretical specific charge are discussed by means of operando XRD and ex-situ TEM analysis. The excellent rate capability enables the use of TiSb2 in NICs, achieving promising high energy densities of 132 Wh·kg-1 at 114 W·kg-1 and 65 Wh·kg-1 at 11 kW·kg-1, which are among the best reported values for alloying materials in NICs. However, due to the well-known problem of volume changes upon cycling of alloying materials, the capacity retention needs to be improved. Using cross-linked functional binders as CMC-PAA we enhanced the retention after 1000 cycles from 10% to 63%, paving the way to develop new high-performance anodes for NICs.
INTRODUCTION Nowadays the main research on electrical energy storage (EES) systems is focused on batteries and supercapacitors. The fast development of portable electronic devices and electric vehicles has increased the need for Li-ion batteries (LIBs) with a significant rise in the Li demand, hence increasing the price of raw materials. Research in sodium-ion batteries (NIBs) has been reconsidered in the battery research community as demonstrated by an increasing number of review papers published covering the whole range of materials 1–4 or focused on negative electrodes 5–8 and electrolytes.9 When high energy density and power density are simultaneously needed, hybridization between battery and supercapacitor electrodes can offer the figures of merit required. These devices are assembled using a batterytype and a capacitor-type electrode.10 Lithium ion capacitors (LICs) are the most commonly known devices, a mature technology at research level 11–18 that reached the market in 2015.19 However, the first sodium ion capacitor (NIC) was not developed until 2012 by Yin and coworkers.20 On the one hand, battery-type electrodes store energy by means of faradic reactions happening in the bulk of the electrode material. In the case of negative electrodes, the low voltage plateau of these reactions allows obtaining higher energy density values than electrical double layer capacitors (EDLCs), but their durability is limited. On the other hand, capacitor-type electrodes, normally activated carbons (ACs) derived from different sources,21 provide high power density and fast charge/discharge as well as long cycle life. Thus, bringing together these two electrodes with different
energy storage mechanisms it is possible to develop a device able to get both systems’ best features, such as high energy density and high power density, as well as being safe and durable.22 Regarding negative electrodes for NIBs, ternary Na-TiO and quaternary Na-Ti-Fe-O 23–27 achieving specific charge values below 200 mAh g-1 and hard carbons,28,29 with an specific charge up to 300 mAh g-1, are the most studied in the recent years. Nevertheless, those materials provide limited energy density. Among the materials for higher energy density applications, Sn- and Sb-based alloys,30,31 are some of the most promising alternatives. Alloying elements can reach high energy density values owing to their high density and their ability to store large amounts of Na ions. However, these materials undergo drastic volume changes upon (de)sodiation.32 Sb is a good candidate to replace metallic Na as negative electrode for NIBs because of its high theoretical specific charge of 660 mAh g-1 and its good performance at high current densities.33 NICs are still in a very early stage of research,34 and thus, in the search of the optimum system different negative//positive electrode configurations are often reported: HC//AC both derived from olive pits,18 peanut shell derived carbons as negative and positive electrodes,35 NanNiCo2O4//AC,36 NaTi2(PO4)3-rGO//AC,37 Na2Ti2O4(OH)2//AC38 and some systems based on alloying materials as Sb2O3-carbon fiber cloth (CFC)//CFC and Sb2S3-CFC//CFC39 and SnS2/GCA//A-KB,40 showing good stability. The energy-to-power values of all above mentioned NICs are plotted in Figure S1.
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In the last years, efforts have been made to improve the cycling stability of Sb-based electrode materials for NIBs by (i) designing nanostructures,41 (ii) tuning the crystalline structure of active materials,42 (iii) dispersing Sb in carbonaceous matrices 43–47 or (iv) synthesizing MSb alloys, where the metal M can be inactive, such as Fe,48–50 Cu,51 Ni 52 or Zn,53,54 or active toward Na such as Bi.55 Titanium antimonide, TiSb2, has been recently studied for LIBs, exhibiting high specific charge values and good cycling stability.56,57 In this work we report on TiSb2, which was synthesized by an easy and scalable solid state route and was evaluated for the first time as anode material for NIBs and NICs in a classic cell configuration.58 The effect of the particle size on the electrochemical performance of this alloy and the selection of the binder for high capacity retention NICs are discussed. MATERIALS AND METHODS Scheme 1 shows the preparation route for the samples studied in this work. The TiSb2 alloy was prepared by placing a mixture of the stoichiometric amounts of Sb (ABCR, 100 mesh, 99.5% purity) and Ti (ABCR, 325 mesh, 98.7% purity) in a tubular oven and heating it at 900° C during 12 h under an Ar flow. The pristine alloy (TiSb2) was milled in a Pulverisette 7 using 5 mm ZrO2 balls in the weight ratio 1:40 during 2h (TiSb2-BM). In a following step, the alloy was carbon coated by ball milling with 7 wt% C-nergyTM Super C65 carbon black (Imerys Graphite & Carbon) in a Fritsch Pulverisette 5 using ZrO2 jars and balls in the weight ratio 1:30 (TiSb2-C).
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between 0.05 - 1.5 V at different current densities, where C is defined as 552 mA g-1. TiSb2-C20h electrodes for hybrid supercapacitors were prepared by using the same procedure described before but with different binders. The electrodes were characterized in a three-electrode Swagelok® system using sodium metal as the reference electrode and commercial oversized AC (YP-80F Kuraray, Japan) as the counter electrode. An oversized counter electrode was used in order to avoid limiting the current supplied to the working electrode. Also, this setup minimizes the cell resistance, leading to a more precise measurement of the real rate capability of TiSb2-C20h electrodes. Full NICs were assembled in three-electrode Swagelok® systems using AC derived from olive pits as positive electrode, TiSb2-C20h as negative electrode and sodium metal as the reference electrode. Galvanostatic charge/discharge measurements were recorded from 1 to 3.6 V at different current densities between 0.05 - 8 A g-1 in a VMP-3 potentiostat (Biologic, France). RESULTS AND DISCUSSION 3.1 Structural and morphological characterization of TiSb2 alloy
Figure 1. X-ray diffractograms of the TiSb2 based samples studied. Pristine TiSb2 and the TiSb2 and Sb patterns have been added for comparison.
Scheme 1. Synthesis route for the TiSb2 based alloys studied. The crystal structure of TiSb2 alloy before and after ball milling process was characterized by X-ray diffraction (XRD) using a Bruker Discover D8 diffractometer (monochromatic Cu K radiation, = 1.5405 Å) and the XRD patterns were refined using FullProf Suite program.59 The morphology, homogeneity and particle size of the powder samples were characterized using Scanning Electron Microscope (SEM) FEI Quanta 200 FEG and Transmission Electron Microscope (TEM) FEI Tecnai G2 F20. Electrochemical measurements were evaluated in half-cells CR2032 coin-cells against metallic sodium (Panreac). For this purpose, the working electrodes were prepared by casting a slurry with composition TiSb2 alloy:Super C65:CMC in ratio 80:10:10 on aluminum foil. The electrode foils were dried overnight at 80ᵒC and cut in circular shape with 12 mm diameter. The cells were assembled in an Ar-filled glove box (O2 and H2O
