High performance titanium antimonide TiSb2 alloy for Na-ion Batteries

Article Cite This: Chem. Mater. 2018, 30, 8155−8163

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High Performance Titanium Antimonide TiSb2 Alloy for Na-Ion Batteries and Capacitors ́ ez-Cam ́ er,*,†,§ Jon Ajuria,† Francisco Bonilla,† Begoña Acebedo,† María Arnaiz,†,§ Juan Luis Gom † ‡ María Jaú regui, Eider Goikolea, Montserrat Galceran,† and Teofí lo Rojo*,†,‡ CIC energiGUNE, Albert Einstein 48, Technology Park of Á lava, 01510 Miñano, Á lava, Basque Country, Spain Inorganic Chemistry Department, University of the Basque Country UPV/EHU, P.O. Box 644, 48080 Leioa, Spain

† ‡

Chem. Mater. 2018.30:8155-8163. Downloaded from pubs.acs.org by UNIV OF RHODE ISLAND on 11/30/18. For personal use only.

S Supporting Information *

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 crosslinked functional binders as carboxymethyl cellulose−poly(acrylic acid) we enhanced the retention after 1000 cycles from 10% to 63%, paving the way to develop new high-performance anodes for NICs.

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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 materials1−4 or focused on negative electrodes5−8 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 battery-type and a capacitor-type electrode.10 Lithium ion capacitors (LICs) are the most commonly known devices, a mature technology at research level11−18 that reached the market in 2015.19 However, the first sodium ion capacitor (NIC) was not developed until 2012 by Yin and co-workers.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/ © 2018 American Chemical Society

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−Ti−O and quaternary Na−Ti−Fe−O23−27 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 alloys30,31 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 Received: June 22, 2018 Revised: October 19, 2018 Published: October 29, 2018 8155

DOI: 10.1021/acs.chemmater.8b02639 Chem. Mater. 2018, 30, 8155−8163

Article

Chemistry of Materials Scheme 1. Synthesis Route for the TiSb2 Based Alloys Studied

negative and positive electrodes,35 NanNiCo2O4//AC,36 NaTi 2 (PO 4 ) 3 -rGO//AC, 37 Na 2 Ti 2 O 4 (OH) 2 //AC, 38 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. 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 M−Sb 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.

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in a voltage window 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).

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RESULTS AND DISCUSSION 1. Structural and Morphological Characterization of TiSb2 Alloy. Figure 1 shows the XRD corresponding to TiSb2

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 2 h (TiSb2-BM). In a following step, the alloy was carbon coated by ball milling with 7 wt % C-nergy 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). The crystal structure of TiSb2 alloy before and after the ball milling process was characterized by X-ray diffraction (XRD) using a Bruker Discover D8 diffractometer (monochromatic Cu Kα radiation, λ = 1.5405 Å), and the XRD patterns were refined using the 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-cell 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 80 °C and cut in circular shape with 12 mm diameter. The cells were assembled in an Ar-filled glovebox (O2 and H2O < 0.1 ppm) using metallic sodium as counter and reference electrodes, and two Whatman GF/D borosilicate glass fiber sheets as separator, and the electrolyte was 1 M NaClO4 in ethylene carbonate:propylene carbonate, 1:1 (EC:PC, w/w), with 5 wt % of monofluorethylene carbonate (FEC) as additive. Galvanostatic measurements were performed in a Maccor Series 4000 potentiostat

Figure 1. X-ray diffractograms of the TiSb2 based samples studied. Pristine TiSb2 and the TiSb2 and Sb patterns have been added for comparison.

based materials used in this study, all indexed in a tetragonal system I4/mcm with the lattice parameters shown in Table 1. A secondary crystalline phase that corresponds to Sb appears after the ball milling process. The amount of crystalline Sb, calculated from Rietveld refinement (Figure S2), was 18% for TiSb2-BM and 8 and 21% for the TiSb2 samples carbon coated during 30 min and 20 h (TiSb2-C30m and TiSb2-C20h, 8156


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Chemistry of Materials

and composites.48,49,51,61 The consecutive charge/discharge (sodiation/desodiation) profiles of all samples resemble those reported by Qian et al. for their nanostructured Sb composite, with a two-step sodiation/desodiation process.62 Figure S3 shows a comparison of the cycling behavior of TiSb2 and TiSb2-BM samples cycled at C/20 and C/10 vs Na+/Na. The large TiSb2 particles were not able to reach more than 140 mAh g−1 even at the slower cycling rate. The submicrometer sized TiSb2-BM particles exhibited very similar behavior at both current densities, delivering an initial capacity around 275 mAh g−1 in charge (sodiation) and 150 mAh g−1 in discharge (desodiation). After an activation of 20 cycles these electrodes reached 200−210 mAh g−1, maintaining about 190 mAh g−1 with little fading until 100 cycles but still very far from their theoretical capacity. In order to enhance the electrochemical response, electrodes of carbon coated TiSb2 alloy were tested (Figure 4). The milling time has a strong impact on the composition, as concluded from XRD refinement. After coating with a 7 wt % of carbon during 30 min, the TiSb2 structure remained as the main component with small crystalline Sb fraction (8%). The TiSb2-C30m sample exhibited slightly better electrochemical behavior than the bare ball milled alloy, starting at 276/176 mAh g−1 in the initial cycle and showing good cycling stability, with a slight increase up to 220 mAh g−1 at the 100th cycle. The Coulombic efficiency was improved from 53% to 64% in the first cycle and a stabilized value after 10 cycles of 99% for the 98% obtained by TiSb2-BM electrodes. These electrodes were able to retain more than 80% of their initial discharge (desodiation) capacity for about 285 cycles. Likewise, the alloy coated with 7% carbon during 20 h, TiSb2-C20h, exhibited improved electrochemical behavior. This sample delivered higher specific charge in the first charge and discharge cycle, 405 and 198 mAh g−1, respectively, and reached 225 mAh g−1 after 20 cycles. Much better capacity retention was observed for these electrodes, maintaining more than 80% of their initial discharge capacity for around 335 cycles. The initial Coulombic efficiency was only 49%, indicating that a larger amount of sodium is trapped in the electrode, but stabilized at 99% after 20 cycles. Since the amount of carbon is the same compared to the alloy milled during 30 min, the contribution coming from the amorphous carbon coating should be similar. The higher irreversible charge loss in the initial cycles could be a result of the smaller particle size and, therefore, larger area and larger solid electrolyte interphase (SEI) formation extent.

Table 1. Unit Cell Parameters and Crystallite Sizes of the Refined TiSb2 Based Alloys Obtained by Rietveld Method sample

a (Å)

c (Å)

volume (Å3)

TiSb2 TiSb2-BM TiSb2-C30m TiSb2-C20h

6.6497(4) 6.65208(4) 6.656(1) 6.646(4)

5.8076(4) 5.8040(4) 5.806(2) 5.797(6)

256.80(1) 256.8(1) 257.27(4) 256.07(8)

crystallite size (nm) by refinement 686 17 18 a

a

Could not be calculated.

respectively). The calculation is based on the observed XRD peaks; thus, possible contributions from amorphous phases are not taken into account. SEM images are shown in the insets of Figure S2. The solid state synthesis leads to large particles with sizes between a few and several tenths of micrometers fused in large agglomerates. After the ball milling processes, particle sizes are reduced to the submicrometer range between 100 and 800 nm, approximately, agglomerated in few micrometers sized grains. Crystallite sizes, calculated from Rietveld refinement, decrease from almost 690 nm in the pristine alloy to around 17 nm after milling (Table 1). TEM images of the TiSb2-C20h sample confirmed the presence of nanometer sized crystalline domains after ball milling and carbon coating processes (Figure 2). The distance observed between the lattice fringes (Figure 2b) corresponds to the (200) plane of the TiSb2 structure. Moreover, TEM images revealed that both larger and smaller particles have a continuous carbon coating with thickness ranging from around 2 to 20 nm (Figure 2b,c). 2. TiSb2 as a Na-Ion Battery Anode. The first, second, and fifth galvanostatic cycles of the TiSb2-BM, TiSb2-C30m, and TiSb2-C20h samples measured at C/10 rate are shown in Figure 3. The first cycle sodiation profiles of TiSb2-BM and TiSb2-C30m samples show a sloping plateau, both at higher potential than a similar transition metal antimonide, FeSb2.60 In the case of TiSb2-C20h, the presence of a higher amount of crystalline Sb (21%), and possibly some amorphous Sb, could be responsible for the more positive potential of this sample in the first cycle, 50 to 100 mV higher. This could be due to its smaller particle size and to the better distribution of carbon. The specific charges reached in the first sodiation of all samples, 284, 276, and 405 mAh g−1, respectively, are lower than the theoretical specific charge of 552 mAh g−1, assuming a similar reaction mechanism to that reported for similar alloys

Figure 2. TEM images showing carbon coated TiSb2-C20h alloy particles with crystalline nanodomains and continuous carbon coating (a and b). Colored STEM image of a representative carbon coated particle (c). The carbon mesh corresponds to the sample holder. The alloy is in blue and carbon in red. 8157


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Figure 3. Galvanostatic charge/discharge curves corresponding to the first (black lines), second (red lines), and fifth (blue lines) cycles of TiSb2BM (a), TiSb2-C30m (b), and TiSb2-C20h alloy samples (c).

achieved, the first charge/discharge cycle of the TiSb2-C20h alloy was studied by operando XRD (Figure 5). At the beginning of charge, all peaks correspond to crystalline TiSb2 and Sb phases. No peak shift is observed for both phases, and the intensities remain virtually constant, with little decrease, until the beginning of the plateau at ca. 0.5 V. The Sb phase clearly disappears (see peaks at 33.2°, 46.5°, and 48.8°) as new peaks corresponding to hexagonal Na3Sb phase appear at 24.7°, 38.9°, and 39.9°. Those two last overlap with the most intense TiSb2 peak at 39.6°, which does not seem to disappear since two peaksof Na3Sbare not clearly observed. The cyclic voltammetry of sample TiSb2-C20h (Figure S4) confirms the aforementioned mechanism. During these in operando measurements unreacted TiSb2 remains even after the first cycle. According to Baggetto et al.,63 the lack of full reaction observed for a η-Cu6Sn5 alloy, which shows around 32% of its theoretical capacity, is attributed to the steric hindrance of Na ion diffusion inside the structure. Moreover, they estimated an optimum alloy particle size of about 10 nm to fully utilize its capacity. They also proposed a mixture of η-Cu6Sn5 and Sn in 66/34 wt % ratio to increase the reversible storage capacity. Similarly, we observed lower capacity than expected for TiSb2 alloy, around

Figure 4. Specific charge and Coulombic efficiency of TiSb2-BM, TiSb2-C30m, and TiSb2-C20h samples.

Despite of the improved cycling stability resulting from long time carbon coating, the specific charge is still lower than that expected from the proposed reaction mechanism. To better understand the reasons why a complete reaction is not

Figure 5. Operando evolution of the XRD pattern recorded at C/30 rate (right) and the corresponding voltage profile (left). 8158


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not only stabilizes the electrode surface but also eliminates electrode mechanical stress during cycling.64 Poly(vinylidene fluoride) (PVdF) has been widely used in commercial batteries owing to its good chemical, electrochemical, and thermal stability.65 Nevertheless, it is relatively expensive and needs the use of volatile organic solvents to make the slurries, such as N-methyl pyrrolidone (NMP), that are toxic and expensive.66 Moreover, the poor flexibility of PVdF cannot face the large volume changes of alloying reactions.67 In contrast, alternative water-soluble, environmentally friendly, and low-cost natural polymers, such as carboxymethyl cellulose (CMC), sodium alginate (Na-Alg), or poly(acrylic acid) (PAA), have been recently introduced as binders.68,69 All these binders are characterized by their high cross-linking nature derived from the hydrogen bonds created between the binder’s carboxymethyl groups and the hydroxyl groups of the active material’s surface.70 These bonds create a matrix where the active material can uniformly accommodate, and it allows mitigating the mechanical stress induced by the large volume changes, thus increasing the capacity retention of the active material and providing an overall better electrochemical performance.66,71−73 Na-Alg is more polar than CMC polymer chains and so can ensure better interfacial interaction between the binder and the active material or PAA. Moreover, Na-Alg has a higher crosslinkage network than both others and can be combined with CMC to obtain excellent electrochemical properties.64 For that reason, CMC, Na-Alg, and combinations between CMC-Alg and CMC-PAA were selected to set these different TiSb2-C20h battery-type electrodes for the NICs. Figure 7 shows the rate capability study of TiSb2-C20h electrodes prepared using the above-mentioned binder

40% of the theoretical value, and persistent unreacted cores. Figure 6 confirms by local STEM and semiquantitative EDX

Figure 6. (a) STEM image of a representative TiSb2 alloy particle in the charged state (sodiated), (b) zoom in on the edge of the particle in (a), and (c) zoom in on the edge of the particle in (b); the image has been colored according to the Na ion concentrations found by EDX, ranging from yellow (more Na) to blue (less Na). Local EDX quantification results: point 1, (Na:Ti:Sb) = 0.6:1:1.3, and point 2, (Na:Ti:Sb) = 0.3:1:1.8, where the concentrations have been normalized with respect to the Ti atomic %.

analysis performed on charged electrodes that the higher concentration of Na ions is found in the outer shell of the particles, whereas in the core the concentration is much lower. During discharge, Na3Sb is transformed to amorphous Sb. Since part of the TiSb2 phase remained unreacted and never disappears, it is difficult to confirm whether the reverse reaction that forms crystalline TiSb2 from Ti and Na3Sb upon desodiation would take place. To fully understand the reaction mechanism and obtain the maximum reversible capacity, higher resolution data is needed. Nanometer sized TiSb2 particles should achieve specific capacities closer to the theoretical values; however, finding the proper precursors might be an issue. Despite TiSb2-C20h based electrodes with submicrometric particle size possibly not showing advantages in terms of specific charge with respect to pure antimony as negative electrodes for sodium ion batteries, they show stable cycling properties and lower expected polarization,48 especially at high rates. These electrodes delivered around 100 mAh g−1 at a current density as high as 5.5 A g−1 (10C), as it is shown in Figure S5, higher than the capacity observed for pure Sb−C electrodes treated following the same procedure. Moreover, at such high current density the reaction potential might be still high enough to avoid Na plating (inset Figure S5). The promising properties found at high current densities and its low working potential vs Na+/Na (∼0.4 V) are encouraging to test TiSb2-C20h based electrodes as negative electrodes in NICs. 3. TiSb2 as Na-Ion Hybrid Capacitor Anode. One of the main challenges of hybrid supercapacitors is to maintain the excellent cycle life of EDLCs. Thus, an accurate electrode design is needed to obtain a NIC that performs as good as an EDLC in the high power region. It has been previously demonstrated that the Na storage performance is affected by the binder selection, and so it is necessary to use a material that

Figure 7. TiSb2-C20h rate capability between 0.05 and 1.5 V vs Na + /Na using different binders.

formulations, between 0.05 and 1.5 V vs Na+/Na from C/5 to 20C. As it is noted, for all C-rates the electrodes prepared with Na-Alg are the ones that perform the worst, delivering 185 mAh g−1 at 1C and 55 mAh g−1 at 10C, while the electrodes prepared with CMC exhibit 205 mAh g−1 at 1C and 88 mAh g−1 at 10C. Instead, the electrodes prepared with the combination of CMC-Alg and CMC-PAA, owing to their higher cross-linkage network, show better electrochemical 8159


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Figure 8. Galvanostatic charge/discharge profiles of the positive electrode (AC, dash red line), the negative electrode (TiSb2-C20h, dashed-dotted blue line), and the full cell (straight black line) of each NIC.

performance providing 248 mAh g−1 and 235 mAh g−1 at 1C and 81 mAh g−1 and 88 mAh g−1 at 10C, respectively. Despite that at low rates it is clear that the use of CMC-Alg and CMCPAA is better than the other binders, at higher rates there are no significant differences. Thus, combining all CMC-based negative electrodes with an AC derived from olive pits as the positive electrode, which was prepared by casting a slurry with composition AC:PVdF ratio 95:5 on aluminum foil, three different NICs were assembled, hereafter denoted as NIC (CMC), NIC (CMC-Alg), and NIC (CMC-PAA). The mass balance for all the systems was set to be 1:1 based on the capacity vs current density analysis shown in Figure S6. The capacity of both materials diverges along the applied current density values, and thus, it is not possible to define a suitable mass balance for the whole range. Since NICs are designed to be used at high power applications and long cyclability is demanded, a conservative mass ratio of 1:1 was set partially limiting the capacity usage of the TiSb2-C20h alloy in order to increase the durability of the system. With the aim of maximizing the output voltage of all studied NICs, each electrode was preconditioned. TiSb2-C20h (CMC), TiSb2-C20h (CMC-Alg), and TiSb2-C20h (CMCPAA) were cycled between 0.05 and 1.5 V vs Na+/Na at C/10 for 5 cycles in order to form the SEI and supply sufficient sodium to compensate the first cycle irreversibility. For the AC, it was charged to a cutoff potential of 4.2 V vs Na+/Na at 10 mA g−1. Then, galvanostatic charge/discharge curves for the NICs were recorded between 1 and 3.6 V at different current densities between 0.05−8 A g−1. Figure 8 shows the galvanostatic profiles of both positive and negative electrodes as well as the profile of the overall NIC. At low current densities, i.e., 0.1 A g−1, within a discharge time of ca. 30 min, the three NIC systems show very low equivalent series resistance (ESR) and, thus, very low voltage drop. In all cases,

the AC shows an ideal symmetric profile owing to its capacitive storage mechanism that swings from about 1.97 to almost 4.14 V vs Na+/Na while TiSb2-C20h (CMC) works in a narrow potential window from ca. 0.97 to 0.54 V vs Na+/Na. A 10-fold increase in the applied current density (1 A g−1) reduces the discharge time to ∼2 min while the ESR increase is negligible and the profiles of each of the electrodes do not suffer remarkable changes. At high current densities, the differences among the various systems are more noticeable. Regarding the negative electrodes, TiSb2-C20h (CMC-Alg) presents much lower polarization than its TiSb2-C20h (CMC) and TiSb2C20h (CMC-PAA) counterparts due to the lower ohmic resistance. Thus, while the discharge time of NIC (CMC-Alg) at 8 A g−1 is 12 s, the discharge times of NIC (CMC) and NIC (CMC-PAA) are 7 and 9 s, respectively. From the above-described NIC profiles, energy and power density values were calculated and represented in an energy-topower Ragone plot (Figure 9a). Numeric values for energy-topower ratios with respect to the discharge time are summarized in Table 2. As it is seen, at the low power density region, i.e., 114 W kg−1, where the discharge time is ∼1 h, the difference in terms of energy density is negligible, ranging from 115 Wh kg−1 to 132 Wh kg−1 for all the different systems. For discharge times within various minutes differences start to arise, but it is at the highest power density region (i.e., ∼20 kW kg−1), within discharge times of few seconds, where the binder effect is clearly notorious. In the latter regime NIC (CMC-Alg) delivers the highest energy density value, 49 Wh kg−1, while NIC (CMC) and NIC (CMC-PAA) store/deliver 27 Wh kg−1 and 37 Wh kg−1, respectively. Thus, the NIC prepared by the mixture of CMC and Na-Alg binder gives the best energy-topower values: more than 6 times the energy density given by its EDLC counterpartbuilt using the same ACfor the entire studied power density region. Nevertheless, to develop a 8160


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to the initial device fabricated with electrodes based on bare CMC. Thus, we report an outstanding NIC based on TiSb2 alloy as negative electrode able to store/deliver energy densities as high as 72 Wh kg−1 and 37 Wh kg−1 at 4500 W kg−1 and 20 625 W kg−1 power densities, respectively, combined with a good capacitance retention of 63% after 1000 cycles.

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CONCLUSIONS Titanium antimonide based electrodes have been explored for the first time as negative electrodes for sodium ion batteries and hybrid capacitors. Electrodes made from submicrometer sized, carbon coated alloy particles delivered stable specific charge of about 225 mAh g−1 over 200 cycles, about half of the theoretical value. The reaction mechanism, investigated by operando XRD, revealed a reaction mechanism similar to that exhibited by analogous transition metal antimonides. However, hindered sodium ion diffusion in the relatively large TiSb2 particles limits the reachable specific charge. Nevertheless, the specific charge values observed for the ball milled and carbon coated TiSb2-C20h alloy at high rates make it a suitable candidate to be used as negative electrode in sodium ion capacitors. To the best of our knowledge, in this work we report the first intermetallic compound-based sodium ion capacitor in literature, showing excellent energy-to-power density ratios. In order to increase the durability of this device a detailed study in the alloy electrode preparation was carried out studying the effect of different binders such as CMC, Alg, CMC-Alg, and CMC-PAA. Through the achieved energy-topower values and mainly based on cyclability tests, the combination of CMC-PAA was found to be the most appropriate binder for this material. The developed NIC (CMC-PAA) is able to store/deliver 118 Wh kg−1 at 114 W kg−1 and 54 Wh kg−1 at 11 kW kg−1, while the capacitance retention after 1000 cycles is 63%, much larger than the 10% of retention measured for the NIC (CMC) counterpart.

Figure 9. (a) Ragone plot with all studied NICs and their EDLC counterpart based on olive pit derived ACs. (b) Capacitance retention of each NIC.

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Table 2. Experimentally Determined Energy and Power Density Values for Each Studied NIC

S Supporting Information *

tdischarge h system NIC (CMC) NIC (CMCAlg) NIC (CMCPAA)

min

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02639. Comparison to the state-of-the-art NICs and structural, morphological, and electrochemical characterization of all samples (PDF)

s

energya

powerb

energya

powerb

energya

powerb

128 132

114 114

67 81

4401 4486

27 49

19760 21052

118

114

72

4500

37

20625

ASSOCIATED CONTENT

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Energy density (Wh kg−1). bPower density (W kg−1).

a

AUTHOR INFORMATION

Corresponding Authors

*(T.R.) E-mail: [email protected]. *(J.L.G.-C.) E-mail: [email protected].

competitive NIC, apart from excellent energy-to-power ratios, a durability approaching that of EDLCs is needed. For that purpose, the selection of an appropriate binder is crucial. Figure 9b shows a comparison of the capacitance retention of the hybrid systems built using different binder formulations. It is observed that after 1000 cycles at a current density of 5 A g−1 (tdischarge = ∼20 s) the systems using CMC-Alg and CMC can only retain 23% and 10% of their initial capacitance while the CMC-PAA based system is able to keep up to 63% of the initial capacitance. Thus, the cross-linking nature of PAA, which provides high interfacial interaction with the active material, allowsin combination with CMCincreasing the capacitance retention of the overall NIC up to 53% compared

ORCID

Juan Luis Gómez-Cámer: 0000-0001-8127-5046 Teófilo Rojo: 0000-0003-2711-8458 Author Contributions §

(M.A. and J.L.G.-C.) These authors contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 8161


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ACKNOWLEDGMENTS The authors would like to thank financial support by the Basque Country Government under the Elkartek 17 and Elkartek 18 programs (Projects CICE2017 and CICE2018) and by the Spanish Ministerio de Economiá y Competitividad (MINECO) through the project AffINIty (ENE2016-75242R). M.A. thanks the Spanish Ministry of Education, Culture and Sport (MECD) for her FPU predoctoral fellowship (FPU15/04876). J.L.G.-C. and M.G. gratefully acknowledge MINECO for the “Juan de la Cierva” (IJCI-2014-20613) and “Ayudas de Formación Posdoctoral” (FPDI-2013-17329) fellowships.

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